i'm told that a panel can just "pop" out

I am a body tech and have been for ten plus years. And I have never seen a bumper come undone from the fenders for no reason. Usually it's from parking curbs or speed bumps. But don't forget that all your bumper is plastic and it just clips on to a plastic retainer behind the bumper on the fender. Really cheap but works good unless damaged. I would check by looking under the car at the bottom of the bumper. Also if this car had damage before you bought it. And they could have put aftermarket parts such as bumper, fender, or bumper brackets. It would also not stay popped in good..

Mine did just that today! I was pulling into the gas station and heard a hideous noise. When I got out I saw my front bumper hanging from the grill area. The plastic clip seemingly popped out of place on its own accord and left the right side of my bumper hanging. I was able to put it back up/on by popping the clips back into place. Now I'm paranoid every time I hit the slightest little bump.