Like quiet quitting—where employees mentally check out of their jobs, instead of taking the financial risk that comes with actually quitting—those who are “quiet vacationing” are similarly tricking bosses into thinking they’re working. 

But instead of wiggling their mouse every now on then so that they appear to be active while secretly binge watching TV, these workers are taking the trend one step further: They’re going on vacation without formally taking leave, and pretending to still be online. 

Essentially, some workers could be scanning through Slack or responding to the odd email from a beach in an entirely different country, right under your nose this summer. 

And it’s your millennial workers you should keep the closest eye on.

Millennials are so scared of looking like they’re slacking they’re quiet vacationing instead

According to a previous report on out of office culture by Harris Poll, 28% of workers are guilty of taking time off work without comminucating it to their employer. 

Despite all the flak they get about being lazy or unprofessional, less than a quarter of Gen Zers have done this—the same as Gen Xers and baby boomers. 

However, nearly 4 in 10 millennial workers have gone on vacation behind their bosses back. They’re also the most likely to have moved their laptop cursor to appear online, or scheduled a late message to look like they’re working overtime.

But all of this comes from a fear of looking like they’re slacking off, the researchers noted. A significant chunk of “quiet vacationers” are likely not using up more leave than they’re actually entitled to, they’re just scared that by asking for those days off work, they’ll be passed up for opportunities.

A separate study from Resume Builder found 43% of “quiet vacationers” are secretly taking up to 3 days off on the company dime, while a quarter are taking the entire work week off.

The researchers echoed that anxiety is the top reason workers are going to such extreme lengths, with 2 in 5 workers worrying about how taking paid time off will impact their job security. 

4 in 10 millennials take a quiet vacation—here’s how to tell if your worker is one of them

The biggest sign your worker is secretly on vacation? A shift in the frequency or timing in their responses is a dead giveaway that they’re preoccupied (or on another time zone), the career coach Kyle Elliott tells Fortune. “If someone who typically responds to emails and Slack messages within minutes suddenly takes hours or starts responding at unusual times, they may be on vacation or working from a different location.”

But, he insists it’s more important employers nip this behaviour in the bud by asking themselves why their workers are feeling the need to lie in the first place.

“This could be a sign of a larger cultural issue, such as a lack of psychological safety or unclear expectations, that needs to be addressed,” Elliott adds.

“Rather than leave employees guessing where and how they should be available, set clear expectations from the get-go. This reduces confusion and ensures everyone knows what’s expected.”