Difficult one, I called out the police when I was worried about some previous neighbours, could hear water and damp patches had appeared on the outside of the house.
Managed to look through the window and the ceiling had collapsed due to the water. Managed to find another guy from up the road who knew how to turn their mains water off from the road.
We called the police as were worried no-one had seen the neighbours for a while and there was washing out in the back yard and their daughter's toys but the police said they couldn't do anything.
They came back a few weeks later and had to move out for about 6 months while the damage was fixed. It was impossible to trace them as turned out they were in a jungle in Nigeria!
Not sure if there is a way you can trip electrics externally without causing problems (esp if there is heating left on in case of pipes freezing etc...).
Was the property owner occupied or let? If let or it's been on the market, agents may have keys.