Anti Thief devices.... what works

On my first '69 Dart back in the very early '70s I mounted a 6" school bell under the battery tray in the engine compartment. It was armed by a key switch that was in the fenderwell over the left front tire. The bell was connected to the dome light switches.
There were several early mornings I forgot about it and got a very rude awakening when I forgot about the bell and unlocked and opened the car door! I imagine it looked like a scene out of a comedy with me fumbling for the keys and trying to find the key switch in the fenderwell while that dang bell was doing its thing.

A big dog works wonders! I used to take my 92-pound Rhodesian Ridgeback with me everywhere I went. I'd leave him in the cab of my Dodge Ram with the windows down. He'd just sit there on the seat quietly waiting for my to come back. It was funny watching people give the truck a wide berth if they were parked in the next spot.