These people will have to scout out the possibilities during daytime - just start writing down number plates etc of any suspicious cars, especially vans. I have started doing that, also if I see someone parking/stopping at the end of our driveway I make a point of going past and looking at them hard. So far so good....
And I would also second that you will need to take out buildings insurance for the time during the build, we did that, as our builder advised us. We did at that time not live on site either. If you can move on in a caravan and get yourself a noisy dog, doesn't need to be big, but it helps.