Tying with padlock

Thread the padlock through both rings and close it, and put the spare key where a bound hand can already reach it.

How it is done

A lock changes the nature of whatever it closes: up to that click there was a tie somebody could work loose, and after it there is a tie that needs one specific small object. So the spare key goes into reach before the lock shuts, not after — on the mattress, in the closed hand, on the floor within sight. A lock is never a reason to have no shears.

What goes wrong with this one

Every locked join needs its key tested in that same lock before it goes near a person — a stiff or wrong key discovered mid-scene is the entire reason spare keys exist.