caution: wet floor

Usually. The question of liability has to do with the application of due care. Posting a sign is one thing, but actually drying the floor and/or blocking off the floor while it dries is a very different matter, and clearly is more of an application of care.

Doesn't really matter whether or not they saw the sign. Liability isn't changed by that in most jurisdictions.

I think that the injured party would have to show that the store was negligent. If the spill just occurred, and the store quickly put the sign down, there is no negligence. If the store put the sign down, but the sign was too small, and the spill was not promptly cleaned up, there would probably be actionable negligence.