You are asking for something that will alert you when you are gone, yes?

The issue here is that you will need whatever it is to alert you to be based outside of the network, ie it will be pinging your network to make sure it is still active... because once your network goes down anything setup within the LAN won't be able to send a signal outside of that :P

I'll think about this for a bit, off the top of my head I can't think of anything that would immediately alert you. There's certainly a lot of easy ways to do the check manually. Why can't you just check once a day? Ie if your goal is to let you know if the plumbers messed up why can't you just call them in the evening and ask them if they got into any wires?