Dave Kerpen was hoping prices would drop on some expensive tickets for a late-September New York Mets game. He kept checking the prices to no avail.

At one point, he checked his StubHub app and added the tickets to his cart. He decided not to buy just then, and closed the app.

Almost right away, he got a phone call. It was StubHub, telling him he could get a 5% discount for the tickets if he bought them immediately over the phone.

“It was surprising because I didn’t even realize they had my phone number,” Kerpen, an author and entrepreneur, told CNBC. “If it startled me, it probably startled most people.”

Creepiness aside, Kerpens said he was disappointed in the meek 5% discount offer and didn’t buy the tickets, but added “it probably works part of the time or they wouldn’t be doing it.”

A StubHub spokeswoman said the company began doing this more than three years ago “for select events” to help consumers with factors like the best seat or the best day to attend an event. She said the practice is to first reach out by email then follow up by phone. Kerpen said he had not received an email.

“When consumers are considering a high cost purchase, there is a desire for a more personal touch, which can give them greater confidence,” StubHub wrote in an emailed statement.

It’s not surprising that a big internet company -StubHub is owned by eBay -would track consumer activity on the web. That’s how brands target users with such increasing precision and how data collection firms build comprehensive profiles that can help advertisers get the messages to the right people at the right time.

But after several years of high-profile scandals over how big internet companies like Facebook collect and use data from users, consumers are paying a lot more attention -especially if they’re not sure how a website got their number.

Last week, comedy writer Ariel Dumas wrote on Twitter that she received a phone call from furniture retailer Wayfair while browsing its website. She wasn’t happy about it.

A Wayfair spokeswoman told CNBC in an emailed statement that the company recently started testing outbound calls to less than 1% of customers “to assist them in the shopping process.”

“We do not make any outbound phone calls based on real-time site activity,” the representative said. “In all cases, customers receive an introductory email from the team prior to any phone outreach.”

Critics have also taken to Twitter to criticize T-Mobile, ePromos, Staples and Liberty Mutual for what they claim is similar behavior. ePromos and Staples didn’t respond to a request for comment. Liberty Mutual did not provide a comment as of press time. T-Mobile declined to comment on the record.

How it happens

Though it may not be clear on the surface for consumers, the practice of figuring out the identity of an online customer, or connecting that person’s behavior to their other online behaviors, is pervasive.

Here’s how it can happen.

1. Many websites work with technologies that help them collect and connect consumer behavior across devices.

There are a bunch of technology companies that specialize in tracking consumer behavior online and in the real world, across different devices.

Wayfair appears to use services from at least one such company, Neustar, which describes itself as a tech company that provides real-time information and analytics for different industries, including marketing. The company says it helps clients know “everything they can about their customers and prospects.”

Wayfair has code from Neustar directly on its website, signifying a direct relationship, according to Ratko Vidakovic, founder and principal of ad tech consultancy AdProfs. It wasn’t clear if the retailer used Neustar in the example the Wayfair visitor tweeted about, though. Neustar initially declined to comment, but after this story’s publication a spokesperson said the company works with Wayfair on the fraud, risk and compliance side. He said unless a person on Wayfair’s website was an existing customer and they authenticated on the website with some form of personal information, Neustar wouldn’t have been involved in this type of experience.

Consumers can turn to a Chrome extension called Ghostery to see what other kinds of tracking technologies sites are using, and block them if desired. Ghostery shows that Wayfair also works with a number of other third-party trackers from providers including retargeting company Criteo and IBM’s analytics company Tealeaf. It also lists so-called trackers from tech giants like Google, Facebook, Snapchat, Microsoft’s LinkedIn and more.

(The practice is not limited to retailers, by the way. Publishers including CNBC use dozens, if not hundreds, of trackers too. For instance, CNBC uses one called Bounce Exchange, whose website says its technology identifies 40% of a site’s anonymous traffic across all their devices and sessions, and Parse.ly to help track traffic to stories.)

2. At some point, the shopper probably entered his phone number into an online form. If that form was on a website that included a tracker, it may have been collected there.

Jeremy Tillman, president and head of product and marketing at Ghostery, said if a consumer has ever added their phone number into a form and there was a tracker on that web page looking for that information, that phone number can be added to a person’s “profile” for those companies collecting that information.

Some companies also aggregate publicly available information to add to these types of profiles.

3. Companies can use technology to figure out when a consumer is on their site, and map it to a phone number already collected.

It isn’t difficult for companies to know who is on their website as they’re browsing -- especially, like in the StubHub example, if the person has identified themselves and put tickets in a shopping cart.

Companies like Neustar along with identity resolution companies LiveRamp and Tapad have the kind of data to map a user, whether they’re anonymous or logged-in, to a phone number, Vidakovic said.

Some companies like Lead Forensics can even provide the names, email addresses and phone numbers of visitors to web sites in real-time.

Creepy or convenient?

Bryan Forbes, a vice president of strategic planning at IN Connected Marketing, said consumers aren’t fully aware of how many methods brands are using online to encourage them to shop.

“The last thing a brand wants to do is scare and turn someone off because they come across as creepy,” he said in an email. But the ways brands can use technology to track people and their behaviors is only getting more sophisticated. As a result, he said there will probably be an increase in “real-time” reach outs based on website browsing under the guise of assisting in the shopping process, despite this risking consumer trust.

“If this become pervasive, what will the consumer [and] shopper backlash be?”

Kerpen, who received the phone call from StubHub, said the company might consider texting consumers before calling. He said bot-delivered texts could probably be cheaper, and might be less intrusive feeling for consumers.

Also, he said if StubHub is going to call, it could at least call with a better deal.