I would anyway,go to the police and make a charge against unknown ( anzeige gegen unbekannt) for stealing the Tom-Tom give them all details they need to know.
You will need the proof that you had reported that to the police to make any claim with your insurance.

My eldest daughter left her cell phone once in the classroom in her desk when she went to the break ( grosse pause) and when she came back it was stolen,the classroom was not shut with a key and it was IMHO neglect of her part,but our insurance paid up and it was no problem.

Hope it works out for you as well!