Kylie Jenner Fans Think Timothée Chalamet's Lurking In The Back Of Pic

Kylie Jenner's boo Timothée Chalamet has been keeping things low-key in their romance ... but if there's one thing about Kylie's fans, they're like hawks unearthing the deets.

The makeup mogul shared two IG mirror selfies Monday, looking all sultry per usual. But hold up -- what's that in the background? Is that ... Timothée, lurking in the shadows?? Well, Kylie's sleuthing fans are 100% convinced it's him!

Of course, it might be a stretch ... but you can't blame fans for trying to catch any glimpse of the elusive couple. After all, with their romance flying under the radar, any hint of them in the same space is striking gold, at least for her fans.

K&T were spotted touching down in L.A. Monday from a trip to the Bahamas -- likely a birthday bash as KJ turned 27 over the weekend.

Kylie shared some birthday cake pics ... with Kris Jenner leaning in with a big grin. But, TC was noticeably MIA from the post.

Before that, they were spotted on a movie date night in June -- a relief for fans who feared they'd thrown in the towel after nearly a year together.

But Kylie's now dished on her hard-won privacy being the key to their romance, so she told British Vogue.