Celebrity branding expert Jeetendr Sehdev -- who's written books on this topic -- came on "TMZ Live" Monday to break down what he thinks has been going on between Syd and Glen, lately. What we've all seen is a lot of sexy body language, and some not-so-happy significant others ... creating speculation their on-screen romance is now an IRL romance.

Check out his take ... essentially, Sehdev suspects this is just good old-fashioned public relations and organic viral marketing -- courtesy of the stars, their teams and the studio behind their film, "Anyone But You."

He makes compelling points. For starters, neither is a huge movie star, at this point ... and having them colead a seemingly run-of-the-mill "hard-R" rom-com probably isn't a recipe for the success Sony's hoping for at the box office.

Yes, they're both hot and on the come-up, but, according to Sehdev, the powers that be probably realized they need a little extra juice to get people interested in the flick. Think "Don't Worry Darling" drama ... but, this time, it's about 2 costars turned would-be-lovers.

If that is, in fact, what's going on ... Sehdev says SS and GP are going about this game all wrong. In his eyes, fans and potential viewers demand transparency these days -- and he feels being more forthcoming about the sitch would go a long way.

The strange thing about this theory is the movie won't come out for a while, as they just wrapped filming. So, any buzz for it now will almost certainly have died out when it eventually hits theaters.