I always hope that the ones I've sold that were just sitting in my drawer are now in full service, working hard. With the exception of a couple old slipjoints with major family/sentimental value, I hate the idea of letting knives sit around unused. I'm no collector, I'm into knives as tools.
When you talk about wondering what your old knives are up to now, I often wonder where my lost/stolen knives are. I can think of a lot of knives over the years that I grew attached to that then "walked off." A Victorinox Explorer that my grandfather gave me (stolen from my dorm in college), an Old Timer Middleman jack I'd traded for in middle school (stolen from my dorm in college), a Buck Squire my dad gave me (stolen from my gym locker in high school), a Case medium stockman that was one of the first non-junk knives I bought with my own money (stolen from my locker during high school football practice), a Victorinox Tinker (confiscated as I boarded a cruise ship - put 'em in your luggage, guys), and so on.