I have twice lost one over past couple of years due to not being careful enough with mask removal. I was charged £70 for replacement each time. (NHS via Specsavers).
Does mean I am very, very careful now. Mask either stays on between different shops, or else I take it off, whilst sitting still on my scooter. I have tried a mask which straps go over t he back of my head, but these are not easy to find.
I am not really surprised that a lost aid was not discovered by cleaners, probably not something they would have even noted when sweeping, or vacuuming an area.