I used to carry my ex RAF dinghy knife (the first, pointed version) in its sheath, bolted inside the folded lid of my crewbag back in the 70s. This was absolutely not in accordance with my airline's policy and in all honesty I don't know whether it would have been of use in a hijacking but it felt a bit safer. It was not found until a dead heading flight on Delta from Bangor to NY, where it showed up on the pax scanner. Even then it took four searches before they finally located it. "What is this for?" "Sharpening pencils!" It was sent for "safe keeping" to the cockpit and the Cpt gave it back to me on disembarking... Having this item in a cockpit, was considered safer than it being in the cabin in those days, when crew members were not subject to security checking.