Depends on particular requirement at that time....it could be a knife or even compact ordnance
like a grenade,or even a simple garotting wire.
You didn't specify anything about a persons skill set, or if its for attack or defence or both, or if its an ordinary civilian situation ,
or it having to be legal, or it having to avoid collateral damage, or if the requirement is for it to be used in stealth, or if the user
even cares about his own survival,
If the only condition given is concealment, consider ALL options.
