I believe that this might be due to a leaking roof but I'm not sure. 

The closest gas stations are 3, 4, and 5 miles away.

. - Submetering - I went through the process for one of my buildings (gas and electric).

My tenant mentioned that there was a leak coming from the toilet in April.

Also crawlspace air also not that good quality if air leaks through those loose ducts.

Other leak along 1st floor ceiling.

Most glaring issues to me are:- DWV pipe leak- Some outlets aren't grounded in both unitsWhat are things you've seen in the past that were real causes for concern/concessions in closing the deal?

BR gets a decent amount of bookings from Floridians so gas prices and opened up air travel would be my immediate concern if you are looking for a cash flowing property now.  

A small leak in the hot tub and decks that could use a good staining are on the list.

And the other things, while you should plan for them with some monthly holdbacks, may only arise every couple years, like a plumbing leak, or an HVAC repair, or leak in the roof.