KK and Tristan were photographed walking out of Nobu Malibu Sunday -- where they were clearly dining together ... even though they appeared to stagger their exits, and ended up leaving in separate cars.

There were lots of other people with them, but none of their children -- which raised alarms among some who thought they were back together ... but we're told that's just not the case.

Sources with direct knowledge tell TMZ ... Khloe and Tristan did, in fact, break bread -- but it wasn't a solo outing for the two of them ... it was actually a get-together for a mutual friend, whom we're told was celebrating a birthday.

Our sources say Khloe and Tristan have some of the same friends, so it's not surprising they'd run into each other like this -- BTW, we're told Corey Gamble and other KarJenners were there as well.

The biggest tell this was really nada is the fact there's no sign of PDA between Khloe and Tristan -- but by the same token, it's understandable why this might've gotten people wondering. We usually only see them together when their kids are involved.

As we all know, Khloe and Tristan called it quits for good in 2022 ... and she's made it crystal clear a romantic reunion isn't in the cards.

Funny enough, Khloe was pretty adamant about saying no to Tristan on the the latest episode of Hulu's "The Kardashians" .... insisting the "door is closed," and that there was no hope they'd get back together, no matter how much time they spend together.

Just like we've been assured before, our sources say Khloe and Tristan are strictly focused on co-parenting at this point -- so it sounds like Khloe's stance then is the same as now ... namely, she's not interested in getting back with the dude.