When I have lost locks (2 times), they appear to have been torn off because part of the zipper was broken as well. In both cases there was nothing missing in the contents and nothing was disturbed (like someone had looked for something).

I assume the locks were caught on something during handling or on the plane itself.

I started using luggage with integrated TSA locks about a year ago. No problems since then. But I didn't have many problems before either.