So use a ballpoint pen on the zipper. You jam it in the nylon teeth so they separate, and you can loot the bag. Afterwards, use the zipper pull to heal the teeth and there's no trace you were in there.
To counter this (and unauthorized use of a TSA key), wrap your bag in industrial cling-wrap plastic.
You should also go right to baggage claim when you land. Most airports don't have someone looking at the bag claim receipts, so thieves can walk right out with a bag.
That's definitely a valid attack on one type of bag but the point stands for everything else: e.g. many people use hard-shell cases for important things, which also radiate “more valuable than socks – steal me”.
In both cases, the problem is the same: the system makes a visible promise of security and has an easy silent failure mode. Getting rid of the TSA “safe” locks or locks at all on zippered bags would improve security by accurately communicating the level risk.
http://www.youtube.com/watch?v=wpIJVWXsBBI
To counter this (and unauthorized use of a TSA key), wrap your bag in industrial cling-wrap plastic.
You should also go right to baggage claim when you land. Most airports don't have someone looking at the bag claim receipts, so thieves can walk right out with a bag.