I've lost several over the years. USUALLY, I can figure out where I had it last and track it down. I've left knives in the ceiling or server rooms of clients, left one in a hotel room, lost a couple while doing scouting activities, and I've sacrificed several over the years to the Lake Gods.
Once I started to really collect, I've made it a rule to order a replacement immediately once I decide that I'm not going to track a particular knife down. That happed just recently when I left a Spyderco Tenacious on a networking installation trip in Ohio. That is the primary reason that I pick inexpensive, readily replacable knives to take on trips, ESPECIALLY boating trips.