I was arriving from AMS at the same time as an arriving flight from FCO and they were directed FCO pax towards control booths whereas we were crossing their path to direct arrivals, so, yes, it would have been possible to slip from one stream to another without it being noticed. It seems to me that, if they want to do spot checks on some flights, they should really do them in the disembarkation corridor (or the end of that corridor) rather than at the base of the pier/main body of the building.