Four years ago the Atlanta TSA rummaged though my bag and nabbed a camera. I was furious. After speaking to the head of Atlanta TSA I learned a few interesting things. Number one that theft is rampant. Atlanta for instance had a large ring that were stealing cameras and selling them on EBay. At another airport TSA employees were so good they good pilfer money out of your wallet as it was going through the scan. I was told by TSA in Atlanta they know it happens, more often than the public might think. There are no cameras in a lot of areas where baggage handlers work. With that being said, if they have access to steal and not get caught, what is to prevent them from adding something to your bag. I have to take my shoes off (And I am ok with that) but handlers have full access.
All it takes to into most zipped and locked bags is a simple pen or pencil Watch the video ! Sure, there are several security counter measures that will show if your bag has been tampered with - but the vast majority of casual travelers don't use them and thus most bags are just easy pickings for the thiefs
TSA will cut off baggage ties AND locks if they are not TSA approved, AND they don't have to necessarily inspect. It happens in stages. As they go though, they cut those off.
Hear it often that luggage gets rifled by baggage handlers is some major airports. Haven't experienced that yet but the only thing of interest in our luggage is my wife's dirty undies. We keep valuable on our person or carry on.
Don't know it where it happened, but I fly out of Philly airport and it is one of the worst in the country. I was robbed once and it most likely happened there on my return from a work trip within the states. Last time I ever put anything good in my checked bag. I have heard from airport workers how easy it is to open a bag, even with locks (see video above), and there are plenty of blind sports in camera areas that workers have an opportunity to steal your stuff. I would think the workers at the Cancun airport are more honest and value their jobs more then the people in the states. I just don't put a lock on my bag. If I was a thief and I saw two bags one locked and one without a lock, you better believe I would cut the lock off, or rip the zipper on the locked bag before I would the bag with no locks. My assumption would be the locked bag has good stuff in it, or why lock it at all.
I never lock my bags. However any thing of value such as jewelry,sunglasses and shoes are always in my carry on. You never trust anyone,anywhere.