I would say it's uncommon for cornsnakes to come out during the day. Some snakes are exceptions. It's more likely for your snake to emerge at dusk. A brightly-colored snake is a target for birds if it is out during the day. A captive cornsnake may become complacent and lounge about out in the open in daylight.
I've also noticed that if I come home after dark, most of the snakes are not out, but after I turn lights on, (lamp lights- pretty low level) they all appear. Is it because they sense activity, or because it's like dusk? I don't know. it's just what they do.