A neighbor (who has a tenant living in their unit as well) 2 floors above's window fell out and crashed onto my tenants' brand new window unit air conditioner and her car, damaging both. 

Also built in an air conditioner, which is a must in Thailand.What was the outcome?

I think that given that the former tenants gave you back the keys and the text message would lead a reasonable person to believe that they abandoned the property left in the unit (much like stuff left in a unit after a lease expires -- it seems like I am always throwing out a stray bag of ratty Christmas decorations that I assume have been abandoned).I think the safe way to proceed would be to text the former tenants with a date and time when those items will be removed from the unit, along with a request for them to return the air conditioner.

Eventually (like once we marry, have kids and need more bedrooms; 2-5 years out), we will move into the main house and rent out the carriage house.There is already a poured concrete pad and hookups outside for an air conditioner, probably a compatible thermostat in home already.

Here in Texas its really the Air Conditioners.  

They're running the air conditioner  / heat too much !

They would not address non-compliance issues with the tenants, as I would handle that myself as the property manager.Prior to Winter the elf would check weatherstrips on doors and windows, install weatherizing faucet covers and storm windows, disconnect garden hoses, disconnect air conditioners, and put them away.

We provide a range and hot water heater and furnace/air conditioner for our rentals.

Financial condition, ability to accept risk, time alloted, expertise all play a role.

He said "she looked like she was loaded" and being a veteran of the Korean war, he wasn't phased much by it.A few years ago another tenant's estranged adult daughter (not living there), broke into her mother's apartment by pushing the window air conditioner into the bedroom and crawling through the window.