Yep...happens all the time. I've had lots of coins & TBs go missing. Some do reappear after time, some don't. Here's 3 of mine that have interesting stories:
This one was forgotten in the bottom of someone's bag for 9 months! Then it was lost again & rediscovered by someone jogging on the road, nowhere near the cache.:
http://www.geocaching.com/track/details.aspx?id=740057
This lil guy was actually lost at a rest stop. Then rediscovered, badly chewed up, almost 2-1/2 yrs later at that same rest stop!
http://www.geocaching.com/track/details.aspx?id=490590
And then, sadly, this one was found destroyed, after traveling 21,627miles (all over the world!):
http://www.geocaching.com/track/details.aspx?id=500094
I even had one POW/MIA geocoin that was stolen right out of the travelbug hotel I placed in my own front yard on its initial placement. Sad, but true.
The best thing you can do, when you visit a cache that lists trackables, yet when you get there, there are none, make a note of it in your log entry. That way the owner of both the cache & the trackable can look over the info if they are watching those items.
Happy Caching!
Tartan Kitty