Greg, this has happened to me so many times! I had a huge mature clownfish and she disappeared. Nowhere to be found on the floor or anything. Recently I had a mated pair of cleaner shirmp, one went missing! And like you, my sixline recently went missing BUT I found him on the floor =-/
I think everyone in the hobby experiences some type of loss where you have no idea where the fish went lol There's just no explanation.
As far as your sixline, I mean he only went missing yesterday so I would wait a day or two but if he hasn't come out to feed when he does normally I'm assuming he's missing.
Good luck!