Protecting garage door entry

We have had some snatch and grab thefts recently. Leave your garage door open, someone goes in, steals bike, rides away. I'm in the back or side of house and never saw it. Want to prevent it. I already have installed a full Ademco Vista 20p system running, but don't want to have to arm it just for this. Have searched, but not found it ready made anywhere. My thinking: Use standard Ademco siren - 12 vdc I think. Use garage door beam kits; e.g. Liftmaster, Genie, Chamberlain.