OT: Child "seeing" spiders and birds

My son frequently talks about seeing birds where there aren't any. He will point at windows or the ceiling of his room and say, "mom! Look at the bird!" This morning he told DH and me that there were spiders in an empty plastic tub, and there weren't any. He was not afraid or alarmed. He often talks about the birds when he wakes up or when he is tired. But he talked about the spiders in the middle of the day when he was fully awake.I am wondering if this is an active imagination or something else.I'll tlak to his doc about it but I am wondering if any of your kids have done this too. He is almost 3. Once again, he has never acted afriad or alarmed by the birds so I doubt they are actual hallucinations.