Vivica A. Fox -- What's Barack Obama Hiding?

Vivica A. Fox told our photog yesterday afternoon ... President Barack Obama smells "fabulous" -- innocuous comment ... or hint of A GIANT COVERUP!?!?!?!?

The "Independence Day" star was at a breakfast with Obama on Thursday and when we asked what kind of cologne the prez wears ... Fox offered up the olfactory praise.

But here's the thing ... even though Obama claims to be off the tobacco wagon, he has yo-yoed when it comes to smoking more times than Oprah's weight. So if he's hitting the cologne bottle ... it could be to mask the smell of his cancer sticks.

So we ask ...