I had the very same thing happen to me, I diden't answer door though, someone else did then got told they asked if mine was for sale as they were looking for a "project car" don't know why as it's good condition and roadworthy, though I am not near you. You are right to be concerned as I thought same as what you did, alarms etc imobilising the car electronically and removing leads etc are all well but this does not stop someone putting it onto back of trailer etc. If you have another car you can block it in with will help.

Martin