Unfortunately, it does happen. We've had multiple things stolen on our private property but are yet to have a trailcam taken (knock on wood).
I have thought about a scenario of someone trespassing and then seeing a flash or the camera itself...and being afraid of being caught on camera for being on our property, that they just take the camera to destroy the evidence. I would rather them come to me and make up a lie that their dog got away or something and that they had to go on the property to get it.
Good luck catching them...it would be even worse if you were about to draw back on a big one and they come walking through the woods that they aren't supposed to be on!