Honestly that sounds legit to me, as that is exactly how the RATP operates (10 or so, "hiding" out of sight behind a corner, checking tickets and issuing fines on the spot). I've been through quite a few of those checks and sometimes they check the ticket or navigo card sometimes they don't, I've also paid a few fines (when I wasn't exactly following the rules) some by cash some by card. It sucks that they acted the way they acted but I wouldn't worry about having been scammed. It really sucks the guy was going through your mothers wallet but if you hadn't paid on the spot, they ask for your personal info, have the fine sent to wherever you live (if you live outside of Europe, you can get away with that I believe) and add 30€ or something to the fine for "processing fees". And if you don't give your personal info, they call the cops. There was a huge anti fraud campaign recently, so these checks are more and more frequent. Unfortunately the system isn't very user friendly which leads to a lot of confused tourists buying the wrong tickets. Hope that incident didn't tarnish your views of Paris too much !