Santa might keep track of who has been naughty or nice, but increasingly fine-tuned marketers track what you're shopping for, and that can mean a pre-Christmas peek at holiday surprises when online ads pop up on shared family computers and devices.
With online shopping set to become the No. 1 choice for holiday buyers this season - consumers spent a record $1.735 billion on Cyber Monday alone - finding a good bargain on the Internet might also squelch the surprise.
"The ever more sophisticated technologies that marketers have for targeting and retargeting means that they're likely to have their message in front of the consumer when the consumer is likely to make a decision," said Susan Bidel, senior analyst at Forrester Research.
And that can spoil the magic.
"It really has taken the fun out of surprising people," said Virginia resident Peggy Chenoweth. "If you share a computer with anybody, it's almost impossible to keep what you're shopping for a secret anymore."
Last year, Chenoweth wanted to surprise her husband and son with a gaming console. She did surreptitious research online and went out at 2 a.m. on Black Friday to buy an Xbox.
"I thought I was keeping a really good secret," said Chenoweth, who works for an advocacy group for people with amputations. The subterfuge only lasted a few hours until her husband got up. "At 9 a.m., he logged onto the computer and did a search and Xbox things started popping up in the sidebar," she said.
"I tried to play it off," Chenoweth said, but the cat was out of the bag. "He was like, 'You got an Xbox, didn't you?'"
Online marketing has been improving steadily, which means more people are likely to find themselves in Chenoweth's position this year. (The National Retail Federation said Monday that nearly half of its surveyed shoppers plan to finish the rest of their holiday shopping online.)
A main culprit is a kind of ad retargeting called programmatic buying, said Clark Fredricksen, vice president at eMarketer.com. "They're essentially following you around," he said.
It works like this. When a shopper goes to a retailer's website and looks at an item, that information is stored and later "read" by advertising technology. That technology triggers the automatic purchase of an ad when the shopper later goes to a page whose owner sells ad space to that advertiser's network.
"It's a technology that's gaining rapid adoption among many, many types of advertisers," Fredricksen said.
Even parents of young kids have to watch out. "My children are 2 and 4, not yet of reading age, but I do have to watch out for photos," Heather Edwards, a stay-at-home mom in Texas, said via email.
In a way, this is a new incarnation of an earlier generation's biggest advertising challenge, said Wharton School marketing professor David Bell.
"Nielsen's monitoring the household but they don't know who's in front of the (TV) box," he said. "It's kind of a parallel problem when you've got a shared device."
Forrester's Bidel said marketers "are challenged to identify who's using what device at what time. They do their best given the tools at hand but we're not down to the point where a marketer can identify exactly who's using what machine at what time."
The reason this creates issues around the holidays in particular is that, most of the year, people shopping online are shopping for themselves.
"Marketers know that those things work, but people are on a very different buying mindset this time of year that may make those messages have a down side," Bell said.
Tracee Figueiredo, a student affairs coordinator in Virginia, said her planned holiday surprise got spoiled this month. Since she and her fiancé are saving up to get married next fall, Figueiredo said she was trying to get him a tablet or a laptop and a watch on the sly, but her recently viewed items list on eBay gave her away when her fiancé used her computer.
"I wish I could have gotten away with it but it was too obvious with the items he wanted obviously glaring him in the face," Figueiredo said. It was a "complete bubble burster."
As digital marketing gets smarter, would-be Santas have to come up with more creative ways to outsmart the parade of ads that can give them away this time of year.
"I won't let me kids touch my laptop because I am afraid they will see an ad or they will see something in my browsing history and ruin a surprise," Facebook user Melissa Foreman-Kenney said on TODAY Money's Facebook page. "My 10-year-old couldn't understand why I wouldn't let him do his typing homework on my laptop yesterday."
Another option is to choose the private browsing setting when doing online research.
Chenoweth said she did most of her online shopping this year at her mother's house, two-and-a-half hours away. For a few last-minute gifts, she said she used her work iPad for research.
"I'm getting ready to make a purchase, and I'm going to go over to my neighbor's and use his computer," she said. "The Internet just makes it impossible .... If you look up almost anything, you have to assume it's going to be public knowledge."
Martha C. White NBC News contributor