As a landlord I have seen just about anything and everything when cleaning out places once a tenant has vacated.
I have had to replace flooring damaged by pets, redone plumbing that was clogged or damaged by irresponsible people, and even had to pay for industrial cleaning in some cases.
This last issue almost had me giving up on they whole process entirely. Sure, I was able to keep the security deposit, but, that didn’t come close to covering the cost involved in repairing the HVAC system. Not only did I have to have the ductwork cleaned due to the stash of drugs we found tucked in the vents, I had to have the unit repaired because they had kept it running non stop with a humidifier set up to help grow the plants in the basement. They had blocked off several of the air vents with cardboard so that the heat could be directed to the growing area more easily. This had causes a build up of debris at the opening of the vents that can only be described as gross. I had to make sure that the HVAC system was clean and safe before I could rent the unit to anyone else. I have changed my rental agreement to include annnual inspections because I want to ensure that tenants are not damaging my proberty like that again in the future. I guess it solves the mystery as to why the previous people never complained about anything or asked me to fix anything. They didn’t want me to see what was really going on in the apartment. I won’t make that mistake again.