WILL NOT WORK. First off, If the army uses it, How would they get that closes to the building, without being noticed and take the time long enough to scan and check the readings THEN plan, They plan about these things hours maybe even days before. And if they're in they're not going to stay still, tell everyone to hold their breath (the enemy would hear it) and scan.
Secondly, For fire-fighters, If it's based on heart-rates, the fire fighters heart's will be pumping after break down doors, yelling, and running with 80 pound gear on.
Lastly, How would you determine pets from humans?