The lock that opened by itself

The locksmith swears the office lock only opens with the right key. There are four right keys, and none of them is where it should be. Some clues in this case come as riddles. If one stumps you, a hint will solve it. Four people and one piece of evidence: one person per row, one per column. Whoever is left alone with it in the same room is the one you're looking for.

Suspects

• Abílio, the Parliament's locksmith: He changed the office lock three times this year. All three times he gave a key to whoever asked.
• Benedita, the night-shift porter: She logs who comes in and who goes out. That night the book stayed blank, for lack of a pen.
• Carmelo, the legal adviser: He says technically nobody went in, because the door was already open.
• Dália, the MP on duty: She was on duty and swears she never left her spot. She won't say which spot.