Fortunately, I heard the pad clatter onto the pavement so I was able to retrieve it. I went home, put the bike back on the stand, and reinserted both pads. I was okay for a short while until I took the NFE on a trail ride on Thanksgiving morning. On the way back my front brake made an awful noise as I slowed while on a long and fast descent. Sure enough, the left pad had fallen out. I had no idea where it was so I limped on home. The nasty noise I heard was the piston hitting the disc.
I put in a new set of pads while paying careful attention and finally figured out what I had done wrong. The pads were not completely inserted into place. I used a small screwdriver to apply pressure and they finally clicked in tight. The rear brake pads did not need as much convincing. So I'm happy I fixed it. But now the front brake surges when I pull the brake lever. I probably damaged the disc when I applied the brakes with one pad missing.