A new friend in my tank?

I have a fish tank with fish in it (obviously) and i was looking to see if all the fish were ok and I sudnly saw a little snail moving on the tank screen, I dunno were it came from because I have neva brought one before...and also I went downstaries after to let my dog out, came back upstaries and he had gone i couldnt find him i looked really hard to find him, has my fish ate him? he was a tiny snail..x