Both ceiling fans and lights in the kitchen and living room were on and I know I shut them off when I left as well as locked the door behind me.

Now for the numbers on the property:Property Description: Duplex with 4BR-2BA total, in a good area within 2 miles of local, large state university; exterior condition is good as it's brick with new metal roof, windows, metal siding on trim; interior has hardwood/tile floors, new ceiling fans, lights, appliances, and paintPurchase Price: $118,000Down Payment: $23,600 (with closing costs credit from seller, final number at closing was right at $24,000)Loan: Commercial loan 15 yr AM, 5 yr balloon, 5.0% interestMortgage: $750/moTaxes: $200/moInsurance: $80/moRent: $2,100 (rented to four college students as one group; tenants pay all utilities)Est maintenance/repairs/cap ex: $300/moVacancy: $100/moProjected cash flow: $670/mo or $8,040/yr (est. $4,200/yr in mortgage principal reduction)Amount spent on repairs: $15,000 (majority spent on new HVAC/duct system and appliances)CoC return: 20.6%I think that covers most of the relevant numbers.  

I began doing floor to ceiling tiled showers, wood like tile floors throughout, brand new kitchen cabinetry, nice laminate countertops, and installing laundry hookups.  

I had an AC clog causing a water leak in the ceiling.

I've seen properties with 12' ceilings in their basement (very nice!)

I have one that when the tenant turns it on, it starts very slow and finally gets up to a decent speed but not as fast as the one that runs in the other non-Master bedroom.I'm looking to buy another one. The current ...

Fix the two points in the attic ceiling (where i pointed out to you) where there are holes.5.

Red flags will differ by investor as one person might not care as much about having to replace plumbing as another, but you should keep an eye out for sloping floors, bad/outdated electrical, plumbing, HVAC, presence of mold, foundation/retaining wall cracks, water damage in the ceilings, and there's probably a few more things but it will differ by location as basements and attics are not things I'd have to worry about in PHX, and scorpions north/south sun exposure are not things you'd probably have to worry about in NY.

I recommend that you look for a home that either has an In-law/ADU already built into it or a part of the home which you can convert to an In-law/ADU (e.g. extra large garage or large storage area with reasonable ceiling height).

Once removed you'll need to replace the plywood and shingles on your roof, drywall the hole in the ceiling, build up new subfloor, and finally patch the flooring you have currently.