The kind  of renter who won’t pay 3$ for a new smoke detector battery or a swap out a light bulb .my  black lab will leave our cool air conditioned house to go lay in the hot sun out on the deck !

As a matter of fact, I consider it "assault and battery with a deadly chemical substance."

One thought, they will require some maintenance - batteries.

The business case for these things is very strong but to add longevity to the alarm I would go ahead and use 9v lithium batteries, the same type that I now use in smoke detectors.

You could rig up a battery power & timed light / radio inside if there is no power on at the property, all goes to help with the look of occupancy.

something that helped me was to get a great PM on your side before you pull the trigger (hence promise them your future business), they will keep you out of the wrong side of town, give you realistic area rents and tell you what's trending, meaning what are young families looking for etc...then after everything is in place go the extra mile and spend time locating a local handy man you can pay yourself independently, typically an older gentleman who works for himself, make sure he's not affiliated with your PM, he'll be the guy you send to look at things to make sure your PM is keeping a tight ship, essentially another set of eyes, who can look at the bigger ticket works orders before you order them...so just tell your PM you have a friend who you like to send over to see the bigger issues...for example: i can tell you that i've avoided $4k HVAC replacement orders several times, given some AC people are dishonest...an issue arose at a house they sent the AC people and they wanted to replace ...but I call my guy before doing anything, so my guy goes in and identifies and even fixes a much simpler issue, in 2 cases replacing the Capacitor ...a $30 battery pack that fits onto the outside Unit...that's just one example...cost me $100 all told, these issues will kill your cash flow, and your PM may simply not have time or bandwidth to double check everything etc...

Batteries from China are not that great, so you can get batteries from here.  

A very simple device, current from a 9v battery makes contact with water and sets of a siren.

For keypads locks you have basically 3 types, mechanical, battery, and internet capable.

And some areas require that the detectors (smoke and CO) be hard-wired (to AC power) AND have battery back-up (which all AC powered units I've seen already have).Become familiar with your local building codes and inspectors.