The windows are dark in this small cottage, it appears unoccupied.
Type: magical; Perception DC 30; Disable Device 30
Trigger location; Reset None
If the residence is entered, a small bell chimes. This bell, via a message spell, is also heard by the local constable, who will arrive in 2d10 rounds to investigate.
By stringing together a basic alarm spell (which causes the bell to go off), messenger and permanency, we have a fantasy equivalent of a modern burglar alarm. Of course, this doesn't have to alert the local constable. It could just as well be used to notify a villain or one of his henchmen. The PCs could also use something like this. You could take this basic effect, and be alerted when somebody enters a location that you are tracking. Once notified, the PCs could rush to the location, scry, or any of a number of actions without having to actually keep physical watch.