Mind my own business.
Another vote for that.
Mind my own business.
As in all situations, it depends on the situation. However, bad guys do carry firearms and they might be up to some bad stuff. Of course, it could be an off-duty police officer. A sober tactical analysis of the terrain and situation is absolutely essential -- i.e., observed demeanor of the person in question, is the person in question alone or with others, what is venue, what are the local CCW/firearms laws, proximity of law enforcement personnel, etc. Also, you must fill-in the blanks on the "what if" check-sheet. Bottom line, extreme caution must be exercised. Simply informing the person in question without careful thought and pre-planning is simply wrong.
Were you in WalMart when I did that?I'd start screaming "Gun!!! Gun!!!!" and running around hysterical while pointing at the guy.
I would discreetly walk up to him, or her, and whisper, "You're showing". The same way I'd let someone know they have toilet paper stuck to their shoe when they're exiting the restroom.