Instead I put an easily breakable cable tie on the zips as a tamper evident seal. The one time I've had a bag come back with it missing has been the time the TSA decided to inspect my bags.
vs a lock, which somebody can open and re-lock and I'd be none the wiser.
As a joke I would put those notes they leave when they inspect your bag in there. I was up to 22 of them when one stickler agent throw them all away except the new one. I was sad. It was fun to imagine them opening that bag every time I flew.
Besides that the TSA has cable ties too. It's not like you can't buy them off the shelve. In your example case the TSA clearly didn't bother spending the time to make it not look like they opened your bag, but it isn't a very reliable way to see if they opened your bag.