A few weeks into the tenancy something strange begins to happen-- I am suddenly hearing very alarming arguments between the two of them from next door (which is saying something as noise does not find its way into my house easily).

Save your texts and such with your tenant and your calls to the police— build a record of your neighbor’s noise.

I would also like to get one of those devices to monitor for smoke and noise (not sure what to get for this either)  What recommendations do you have for a deadbolt keypad that I can change out the code for each new guest?  

First of all, the nuisance stuff you mention like parking and noise usually already have town codes to handle that. 

The 'worry' of something happening would rattle my chi. 

I kind of think this really won’t have any noticeable effect on prices tbh, and frankly even a 2% reduction in net fees is so minor in the grand scheme of things, I mean 2% movements are basically random statistical noise at this point.

Quote from @Jess Keller:  Unfortunately your sign doesn't actually do anything to prevent excessive noise, parking problems, fireworks, property maintenance, trash, pets, unpredictable neighborhoods or anything else related...I am for allowing STRs in mixed used zones and against allowing them in residential zones.

What I love about ADU is if it's separate there is more privacy and less noise transfer.

I have called the police on him for noise ordinance at 3am and called the police on the trespassers.

Whether it’s TV’s a whole bunch of magazines, board games, decks of cards, Frommer's books about the area, whatever, have something for your guests to do while they’re stuck indoors on a rainy day. 6 - Rules about NOISE.