Sydney Sweeney Parties in Ibiza, Fiancé Laying Low Behind the Scenes

4:13 PM PT -- As it turns out, Jonathan was, in fact, with Sydney during this trip ... he was just laying low. Dude popped up on other people's photo dumps ... but not hers.

A little camera shy, eh?

Sydney Sweeney seems to be living her best life this summer -- but the question still kinda remains after this latest development ... is she living her best *single* life???

The actress posted a massive photo dump that shows her partying and enjoying herself during a recent trip to Ibiza -- where she's seen hanging out with friends and hitting the town ... be it in formal-wear, or in a bikini. Indeed, she was rockin' all sorts of looks.

Take a peek at the pics yourself ... Sydney's cheesin' from ear to ear just about every one of them, documenting everything she did -- whether it was poolside loungin, boat-ride sightseeing, surfing or dancing the night away at a concert. SS did all of it and then some.

Obviously, nothing wrong with any of this on its face -- but what becomes evident quite quickly is that her fiancé, Jonathan Davino, is nowhere to be found in the festivities. There is one guy featured who has a passing resemblance to Jon, but it's another dude altogether.

In fact, in some of these shots ... it looks like her engagement ring is also missing -- but that could be explained by the setting some of these pictures were taken in ... including at the pool, where one might presumably ditch such hardware to take a dip.

Point is ... it's interesting that JD is absent from this trip, but not necessarily a smoking gun of her relationship status. Sydney was spotted out on a date night with Jonathan just a few weeks ago in NYC -- where they seemed to be in good spirits and going strong.

She and Jon have actually surfaced a few times over the past several months -- this amid speculation something was going on between her and costar Glen Powell ... whom she shot a movie with, and who looked cozy next to her during a Spring promotional tour.

As far as we know ... she's still on with Jonathan, but man -- Sydney sure does seem unattached here. Hot girl summer, anyone??? 🥵