HELP - how to avoid being robbed during power interruption?

When thieves cut the power to your house. What will happen ?
The UPS will take over the power supply. But for how long ?
If you are in holiday. They can wait some days before they return and empty your house. They can even take your security system and leave no trace behind.
You will be not notified by your security system. All will run till the UPS is drained. Than the modem will stop and leave you without any notification.
If there existed a power-interruption-sensor, the problem would be solved. But without that sensor?
Does someone knows how to work around this system thread?
Ronny De Gussem