It's not only TSA.
It's everywhere like that.
After working for over 10 years as a flight attendant, I have seen and heard a lot.
Most bagage loaders are honest but some are big thieves. In the terminals and bagage sort areas in the airport are security camaras but not in the bagage hold of the aircraft. Whilst loading the aircraft, they often go through people's bags.
And a lock on your bag won't help because they carry small plyers to cut them. Most of those locks can be opened with any key or an aircraft MRT. I often had Pax that lost their key of their handluggage and asked me to help. With an MRT, I could open 90% of them.
The safest way to avoid theft from your bag is to get them wrapped in plastic. (it takes too much time to unwrap a bag so they open an easier 'target'. ) At Lima airport they have that facility. You pay a bit of money (around $10) for it but if you have expenisive clothes or other expensive items in your bag, it's worth the investment.
Also crew often steals. If you forget something expensive like a camera or your wallet, the chance is big that the crew ''hasn't seen it.''
The normal procedure is that crewmember hand it over to the dispatcher of the airline or handling agent. Most crewmember do that but often the dispatcher or handling agent 'forget' to bring it to the office to file the lost property.
Another favourite activity by crew is the following: after the PAX disembarked, crew member do a security check. During take off, coins roll out of passengers pants pockets and end up behind the seat cushion.
After the flight, the crew 'collects' them. We called it 'tips'.