I don't normally check out the state of a bed mattress, but a number of pointers in this place made me think to do it.
A downstairs light switch that had not worked in the dark hallway, a toilet which was brown from limescale buildup and smelled of pee, bathroom tiles which were lined with black mold, hair on the pillows, a broken fridge door, gaps in the flooring.
It turned out I was right to check the mattress - the stains were so disgusting that nothing would have tempted me to sleep on it. Most places with an eye to cleanliness (indeed most homes) will have a washable mattress cover and pillow liners beneath sheets. This bed did not. I reached out to the owner before messaging airbnb but got no response. I was refunded and did not sleep there.