| I think the description so far is too vague to guess at the type of snail it might be, although odds are good it could be a small cerith as they are common hitchhikers.
But a picture, or a least a comparison to a picture is needed. You mention it looking like Cerith #2, so that is good, but there are MANY other genus of snails that have that same general shape of shell. Most of them good, but some bad.
A couple of questions that might help:
1. Can you see any visible eyes when the snail is cruising around the tank (with tiny snails like this you need to find them when they are on the glass).
2. Do you see the snail grazing on the rocks? Do they appear to be attempting to eat algae or something on the surface of the rocks?
3. Do you see a mouth actively trying to feed on the glass/rock?
Brian | 
