This was in the Saturday edition of Le Parisien.
Don't be surprised if you toss your validated ticket or pass and someone comes up to you - with proper insignia, of course, and a hand-held machine that spits out a paper fine. You are free to pay it via debit/credit card or send it in by cheque.
The plainclothes agents are fighting fire with fire, because fraudsters are costing the City too much money.
I was on the Bus yesterday, and was checked, as everyone else was. Some were fined.
So - validate your ticket, Navigo, or pass and keep it till you are out on the street.
Don't get on in the middle of the bus without a valid ticket - you'll probably get caught.