Need more specifics to provide a valid opinion, but if you think someone is abusing the system it is well worth getting to the bottom of the facts and calling your airline's JS Coordinator / Comm. Chair. They could provide you with excellent SA and follow up effectively on the situation.

There were some "forward thinking" operators that were ferrying crews around by having them use other airlines' jumpseats and that was IMHO, wrong. Airlines should either have an agreement (like many Delta Connection Carriers do with DAL) or buy a ticket when they need to move people around for their operations. When this happened it was pretty obvious, as a whole crew would show up at the same time.