I wonder whether relying on actions of bystanders could solve the problem.
I remember a homemade car alarm from the 90ties that had a very amusing alerting mechanism. If the alarm came to the conclusion that the car is stolen, it would project an "I am stolen, please call the police" banner onto the boot cover of the car _only while the car was moving_ (so that the thief would not notice that themselves).
I wonder if we could make something that alerted bystanders _only_. Identifying bystanders is not trivial, but there are heuristics that could help: a phone that is seen in the vicinity for the first time during daylight hours, after a day or so has passed since the presumed time of theft probably belongs to a bystander. The harder part is what to actually tell them (both: what to expect of them and how to identify what is the object that's suspected of being stolen).