is this suspicious?

We are looking to buy a home we have our eyes on and have already gone through inspection. House is in pretty good condition, A few things need fixing like every home. However the home inspector didn’t check out the septic tank. Come to find out, the septic tank is in the long walkway leading to our door, covered in concrete. This septic tank hasn’t been serviced in 7 years, and the seller decided to cover it in concrete. Who does that? I try giving the benefit of the doubt thinking it’s for aesthetic purposes, but even then… It still would have to be serviced. The home is paid off completely by them, And they have invested into a new AC unit, and have fixed plenty of things around the home. But yet the seller doesn’t want to pay for the concrete to be uplifted and tank serviced. Are they hiding something?