Zip Ties! When we got home from WDW in October, DH found a note in his luggage from the TSA stating that his bag was opened. If he had not seen the note, we wouldn't had realized that they had opened the bag because they had placed another zip tie on the bag to "lock" it. (We had spares ties in the luggage.) I thought that was a nice gesture for TSA to do so.
My DH had packed his electric razor and charger in the suitcase so that is probably why the bag was opened.
I was going to buy the TSA approved locks but I thought the zip ties worked just as well.