It probably makes more sense to invest it than to pay off the mortgage but I can definitely see why someone would want to get rid of their debt. 

You may also see a discount fee which means you are buying down your mortgage interest rate in exchange for higher upfront cost.

They also have deadbolts now that you could install on every door which we have contemplated, but it's cheaper to buy lockboxes and only use them for the vacant units than buy a new lock for every single unit we manage. 

FHA loans will have permanent mortgage insurance unless you put down 10%, at which case the mortgage insurance can be cancelled after 11 years.

I recommend having your deals secured by a mortgage and a promissory note for your investors.

Not just because I live there but because it is generally cheaper and better neighborhoods are available.

I just bought a property where my lender gave a mortgage on the purchase with 20% down and a second mortgage on some stuff.

The vacant lot owner can't do anything with that land, so they should be willing to sell it for cheaper than market rate.

As long as you have have enough equity in the deal (20-25%) you could do a rate & term refinance and payoff the seller financed mortgage to get a new loan.

I can only speculate but I suspect she either met someone she's moving in with, or decided she wants to move to a cheaper place, nicer place, or is buying a home.