I have had 2 trolling motors stolen... and I've heard of people getting gas motors stolen. We also don't have cameras at our marine (but the dockmaster LIVES at the marina - and he misses very little)... But still, it's not all on him.
I always twist the thumbscrews until they are together, then run a standard padlock through the ends. Like below. It's not perfect, but it's better than nothing. I also won't buy a new motor, only used because I don't want to be out a lot of money if something should happen (because of this I deal with dodgy outboards at times).
Also something I forgot about on my own boat, that is NOT theft, but is just as real. Those of us with outboards, one of the biggest reasons for "losing" your motor is it falls off the support bracket, or worse the bracket breaks, or even worse drop the motor while trying to mount it (while in the water).... Always use a safety line. Just had a friend tell me his story of woe on how he had his bracket break and his brand new 8hp motor is on the lake bottom (yep they are looking into a dive team to get it, but it was in the middle of the lake not in his slip, so it'll be harder to pinpoint).
Very sorry some numbskull stole your motor, and I hope it's recovered for you (but I am a realist and recognize it probably won't be). Consider a lock on the bracket though.