My UAS instructor's knife + sheath came out & slipped down the side of the seat unseen (See Pingdit's pic - the press stud came undone)
Surely, the knife was secured by the length of nylon thread that held it to the patch? No, I thought not! Back to the Sentry - the thread was often snagged by the seat adjusting mechanism which broke the locking cotton and pulled the string away from the patch, leaving the operator with many feet of trailing string (often happened when deploying on a practice crew emergency drill). Completely useless!