Thanks these replies are helpful. We keep noticing him doing things and then eventually discover they're all part of the illness. he also sits hours polishing a pebble he once found on the beach or a small wooden vase he has. That too has become a habit but I recently read that sufferers like to sit and 'fiddle' with something.
To provide your father-in-law with safer things to fiddle with, if you or a relative enjoy sewing, could you maybe make him a "twiddle" cushion or a knee blanket with zips, buttons, buckles, large poppers, worry beads or buttons threaded on a strong cord, fringes, strips of velcro etc. on a base of different fabric textures that feel nice to the touch, like velvet, jumbo cord or fake fur?
Or put together a rummage box of tactile "treasures" that are badly in need of burnishing?
Could the scissors be tied on a short length of tape that is fixed to a table or work surface ("so they can't get lost") so they are long enough to use on the table but not long enough to reach up to his hair (though I suppose he might use the scissors to cut them free of the tape and then cut his hair if he's really determined to practice his hairdressing skills).