Before I took my boat to S. FL, this past fall, I bought one of those battery-powered motion detector chime/alarms and installed (mounted on velcro) at eye-level on the entertainment cabinet, under the, facing the salon door. I also put on the WARNING window decals on the door, for all to see.
I kept the boat on the Harborage /Indian River, where there can be consideral movement in a strong south wind. The alarm never went off accidently, only when you slid the door halfway open. Ear-piecingly loud. I also used it for on-board security, activating it on to monitor the rear of the cabin, while I slept forward. The bateries were changed out every other month. It was cheap ($25), quick and reliable.