I mean worst case scenario is, new subfloor, new drain plumbing, patch flooring (or replace in bathroom if it's sheet flooring). 

Are they responsible for patching the small holes in the walls from their paintings that they hung up?

I have been painting, patching holes, replaced some shedrock, replace door, code the floor, put new carpet, vinyl floor, bath fitter.. etcI called and talked to the building inspector to find out how he found out, and he never answered the question.

However, lawn was not mowed (as requested) because they think the lawn was too soft after placing the new sods on some spots, and some of the Window sills were patched but did not painted.

I found that one part of the ceiling is patched in the garage, but the seller says they have no idea what it is because it was there when they bought the house.Can anyone has idea what it might be?

I then patched & painted the ceiling.

I could not find the transformer anywhere, had wasted too much time so I just took some pliers, cut the wires of the main unit while it beeped, then let out a sigh of relief when I cut the final wire and everything went silent.Trashed it, and patched up the hole with drywall and mud.

Anywhere from laying down some mulch, to repairing concrete steps, to patching some roof flashing.

Call them and thank them for letting you know and you can decide of you want to divulge the info about their recent loss or just say they are having a rough patch...or nothing.

The house is old and hasn’t been worked on probably since it was built beside patching the roof here and there.