What state was this in?
Was it a square sticker of a round one, and where did he place it on the plate?
Some states have orange and red stickers to indicate teenage and/or probationary drivers.
If a random person were doing that, I’d tend to suspect a prank or that I was being targeted somehow.
Usually I kind of like random weird unexplained behavior, or even pranks in good humor, but something about this feels dangerous to me.
I’d ask the woman what the guy looked like.
And I’d be calling my bodyguards to escort my car and/or investigate who was placing orange stickers, and to follow the guy who made the comment in particular.
Or, lacking bodyguards, I might report it to the police and ask if they know anything about it.
And BTW, having anything unofficial on a license plate can get you pulled over in some places. e.g. Florida: “Nothing shall be placed upon the face of a Florida plate except as permitted by law or by rule or regulation of a governmental agency.”