I have never dropped or lost a knife nor have they come lose from my tether. I think you should examine the secureness of your equipment. Talk this over with your LDS.
I carry two knives and one set of shears. I have been entangled once by a crab pot line wrapped around my tank valve. This was when my buddy and I were diving near my marina dock and we became separated. Shears would have not have been able to cut the rope.
I have my BCD blunt knife secured by a retracting lanyard, my other knife is a large one that goes into the built in knife pocket on the vest of my 7mm farmer johns - it is also on a cord. My shears are stored in my BCD pocket with an old bicycle tube over it's blades so it won't accidentally cut my BCD - it's also on a line. In our cold murky Pacific NW waters and wearing heavy duty gloves, it would be easy to drop/lose one. However, each of these teathers/lines can be cut if I need to get them lose.
Oh, I almost forgot - I carry a 4th as well, which is a small stainless folding knife in my BCD pocket. I carry it here especially for areas that you are not allowed to carry a knife while diving because of laws against taking/cutting pretty coral, etc. (Red Sea, Israel - only Guides are allowed/required to carry knives openly, but not all dives have to be taken with guides). There are fishing lines and nets in Israel, and if I need to cut myself out I would do that.
By the way, I would never "smuggle" a knife into an area that forbid them due to them being classified as a weapon. Some countries only allow knives under a certain blade length, others will not allow a knife, but classify a blunt end as a "dive tool". Check before you go.
drdaddy