I believe they are expressing their displeasure with the fact that I have not accommodated some of their recent requests that went far beyond the scope of what is required (like insisting a licensed electrician switch out a smoke detector when we could do it ourselves) and I had to set some boundaries after years of high maintenance for them.

Broken glass lantern by the pool, smoke detector hanging in hallway, broken ceiling light in the bedroom, face plates missing or missing screws.

I didn't know there were detectors for these sorts of things.

This is where smoke/CO detectors, grounding, electric garage door reversers, and other goodies came from.If the municipality performing the U&O calls out an item, Seller is stuck with resolving it either by fixing it or negotiating it with the buyer.

Don’t skip safety stuff either—make sure you’ve got working smoke detectors, fire extinguishers, and a first aid kit.

Landlords can enter a tenant's property for a variety of reasons, including: Making repairs or replacements Responding to a tenant's request Removing unauthorized window coverings Preventing waste of utilities Changing filters, testing, or replacing smoke detector batteries Inspections Helping with an emergency inside the unit Posting a notice of eviction"Research your state laws because some of your comments indicate a lack of understanding of the landlord/tenant laws where you are operating (like threatening the tenants to keep the security deposit if they don't respond to you, I doubt that is actually a valid reason to keep their deposit, or evicting for late payments, or charging the tenants for your warranty repairs because you missed your 11 month inspection window which was your responsibility, and it looks like your lease says you can charge up to 18% interest for unpaid fees, is that actually legal?).

Hey @Sara Valentine - huge fan of these for my rentals. https://www.amazon.com/Detector-Wireless-Notification-Securi...

They do not just stop paying because there is a door knob loss, or a smoke detector missing a battery. 

I have to close, apply for a conditional use verification, get inspection ( requiring wired smoke detectors, other safety features) and get a business license.

Lightbulbs, batteries in smoke detectors is on them and written in the lease.You can really do whatever you want if it is written in the lease contract but bear in mind they don't own the home and won't care on certain things.