I have  6 smoke alarms on the main floor and one in the unfinished basement. 

This just rings alarms in my head but my state's laws might be different from yours. 

I wouldn't let the tenant know that anything alarming is going on here as the tenant may just be planning to pay you rent and the SD on the 1st of the month.

Put a bullhorn in your office or control room if the alarm pad is not triggered, 3. 

One is even cheaper which really set off alarms in my head.

She had the preventer tabs set so it didn't open, and her window alarm went off.

Are their local or state fire codes that would require changes or upgrades (fire alarms, fire excapes, sprinklers, etc.)If you do go ahead with this use, when you get the Property Insurance be sure to check into the ordinances for rebuilding. 

Any idea how much can something like this cost to get a fire alarm installed?

Ask the agents if adding a fire alarm that rings to the fire dept. or central station would help with rates.  

Whether the key was in the house or outside, the thief could still have taken it.