I've been considering a computerized system, but since I'm both underage and not the owner of the house/apartment, I can't put my idea into practice. This idea works with no pets in the house, and goes like this:
The core of the system is a computer, or more specifically a computer program. It has access to a phoneline (hear me out on this one), and some external sensors. On leaving the house, the person has to press a button near the door (on the inside), and the computer keeps count of how many (authorized) people are still in the house. As long the computer thinks that authorized people are still in the house, no action will be taken. Upon re-entering the house, the person has to disactivate the alarm system (either via a hidden button, numeric code entry panel, etc) hence authorizing his entry and preventing him/her from setting off the alarm.
Next requires a bunch of sensitive motion sensors placed (hidden) near valuables in the house. When a motion sensor is set off, it tells the computer, which dials up a phone number of the user's choice (cell, beeper, etc), so the owner knows, and also *maybe* the police. Also triggered is the release of sleeping (or, poisonous, if legal) gas throughout the house, to stop the thief, until the owner dials into the computer to stop the gas.
Eh?