Legal Question in Criminal Law in Florida

It may or may not be a crime of inciting a riot or disorderly conduct, but even if it weren't a crime, you will definitely be detained by the police and who wants that.

Yes, if it appears that you were trying to incite a panic or cause mayhem, even though there is no destructive device inside the briefcase. After all, that's the implication from the activity. The person would conspicuously put a briefcase next to a crowd and then run away is trying to create attention and imply that there is something dangerous in the briefcase. Bad idea and illegal.