I tried for several minutes to find the bidon, but with no luck. It was still dark, and I finally had to give up and head home. The bidon that I dropped was the spare. Luckily the first bidon still had a little water left in it; I was near to finishing my ride; and home wasn’t quite two miles away. To be on the safe side, I rode at a slower pace. Annoying, but necessary to keep from drying myself out too much.
Luckily a reglar bidon doesn't cost too much. I went to the bike shop after work -- and bought two of them. I want a spare in case I have a repeat of today.