There could be mites - a clean and a paint won't kill them. Insecticides, creosote or diatom (longer term solution) will do that. However, I can't think that they'd kill a chicken overnight. I recently discovered my one uncreosoted chicken house was crawling with them but the five young chickens in there seemed fine.
What symptoms did they have (apart from death!)? Could your tenants have given them something poisonous to eat by mistake? Have a chat to the tenants to see what happened yesterday and see if you can find any clues as to what might have been out of the ordinary. I can't think of many diseases that would kill that quickly - even with coccidiosis and similar you have a miserable looking chicken for a couple of days. So it doesn't sound like disease to me.
The only other thing I can think is whether they were so scared by a predator, they got literally scared to death. I have absolutely no idea whether this is possible but I'm racking my brain!
So sorry, it's horrible when it's that sudden,
H