"Or if they want to be subtle then most cases can be opened with a biro to undo the zip and then closed again by sliding the zip backwards and forwards - leaving no sign it's been opened."
As the lock goes through the zip tab, your point is moot.
It's like saying "I can get through the door after the padlock is removed, then close the door and nobody knows I was there", after replacing the padlock you ignored.
No, that's not quite right.
You're saying that you can remove the door hinge pins, open the door and manage to, by closing the door, reinstall the hinge pins.