The case of the missing towel has been solved! (scroll up for original details!)
The house guest swapped out the towel I left on the bed with one in the linen cupboard. He has a favourite yellow towel. Then my son went to the linen cupboard and took the one he swapped, so I could not find it until my son came home from work.
It was pretty boring and mundane in the end. It all hinged on towel colour preferences #towel #mystery