I've been on international flights where we've been waiting on the tarmac to depart and a flight attendent has announced on the PA system that "we're just waiting to have the bags removed of someone who missed their flight".
I believe the current policy is strict for international flights (no bag without a passenger, must remove it if passenger misses flight), but looser on domestic flights. Not 100% sure that's true, but digging in some air-travel forums comes up with airline employees claiming the same thing as of late-2009.
There are lots of cases where baggage gets delayed or waylaid, and so the passenger ends up flying without their luggage. They then send that luggage onwards on the next flight — obviously without the passenger.
Yes, it would be hard to engineer that to happen, so it's not very useful to someone planning to do something nasty with it — but it does mean there can't be a strict no bag without passenger rule.
I can confirm from recent personal experience that on U.S. domestic flights, luggage can and will be carried on the flight without the passenger on board.
1. Flying back to SF from the east coast on AA - got to JFK a trifle late and just missed the check-in deadline - mainly because the lady at the counter took an inordinate time and appeared to be flirting with the guy just ahead of us in line - yes I tried to check-in online but that wasn't possible because my outbound flight was on a different airline. Go figure. In any event, we were put on standby for the next flight but didn't get into the flight. I was resigned to take my luggage back and find a place to stay overnight in NYC when the gate attendant informed me that our bags were going with the flight and I could collect them at the SFO AA office ! We finally got back that night to LAX (after not making it through a wait on another flight) and then managed a Southwest flight back to SFO the next morning and sure enough our bags were waiting for us.
2. Last weekend, we were on a SW flight from SAN to SFO. Flight was delayed because of bad weather, and there was an additional delay close to the destination where we were not cleared for descent. By the time we were allowed to land, they had run out of fuel (!!) and landed the plane in SJC. Unbelievably they fueled up and took off and landed in SFO - about 3 hours later. Thankfully they let anyone who wanted to get off at SJC deplane and that's what we did. Gave us the time to get home, pick up my car from SFO, have lunch, etc. and went back to SFO to pick up our bags soon after the flight eventually landed. So in this case too, the flight took off with our bags and without us.
I've been on international flights where we've been waiting on the tarmac to depart and a flight attendent has announced on the PA system that "we're just waiting to have the bags removed of someone who missed their flight".