I have found that the right dog trained correctly can be quite a deterrent within a fenced area for more menial thieves, the kind that break into your car or steal things out of your yard. Lights bells or whistles will be a waste if you are not in an area where they will attract attention. Sometimes the most direct and simplistic approach can work the best.

Reminds me of a guy I used to work with. He had people doing much the same including peeking in their windows at night. So he took a buch of pieces of 1x's and drove drywall screws through them and set them down, point up, under his groundfloor windows.. His yard was fenced too, so he wired up a motion detector to a contactor which in turn electrified his fence with 220VAC and turned on several large high powered lights on poles. He then put 'No Trespassing' signs up at his property perimeter.

A few nights later his perimeter lights came on and he heard screaming, and the morning after he left the motion light off he found a spots of blood under his windows and a couple of the screwed boards were gone. Needless to say he got a visit from the local police. When they saw his 'NT' signs, they left him alone. In that state, 'NT' means no one is legally allowed to trespass.

True story. Happened in West Michigan near Paw Paw back in the mid 90's.