Mystery of the climbing baby

Yesterday I left A in the downstairs play area while I went outside to help Roy with some yard work. K was taking a nap and D was supposed to be playing in the same room as A. After a bit, I heard A crying so I went in to check it out. She was upstairs in the kitchen. Naturally I interrogated D. He swears up and down that she was downstairs when he went in to the bathroom and upstairs when he came out. There was no one else to help her. So do I believe D and think that A climbed the stairs herself or do I think D is trying to trick me?

Turns out, he was telling the truth. Today A made it half-way up the stairs before D noticed her. She saw us looking at her, got all excited and promptly tipped over backward off the stairs. Apparently she can climb the stairs aaaannnnnd she can fall down the stairs. Good to know.