This depends who it is a new client who's paid a deposit and confirmed hasn't yet not turned up. I would probably send a 'Is everything OK text' a few minutes after the appointment was due to start. No name, no sexiness.
A repeat booker I'd do the same sometimes I ask repeat bookers to confirm, sometimes I leave it as they've made it to one booking so I trust them to let me know if they can't make it.
Unfortunately have recently been stung by this, and he'll have to pay a deposit for next time. The regulars I have after a bit of a prune, are all reliable. It would truly have to be a disaster for them to no show. In which case if I know it's OK to contact them I would, if not I'd leave it until they next got in touch - or I spot the disaster elsewhere. I have found out about sudden unexpected deaths that way.