Learn Russian: Сейф сейчас закрыт, но кто-нибудь отпирал его сегодня? - The safe is currently closed, but did anyone unlock it today?

This is the "annulled result" use of the imperfective aspect. The first clause establishes that the safe is currently closed. If we used the perfective отпер, it would imply the safe was unlocked and is still open. The imperfective отпирал tells us the action took place but its result was reversed (it was unlocked, but then locked again).

Why do we use кто-нибудь instead of кто-то in this question?

In Russian questions, the -нибудь suffix is used when you genuinely don't know if the person exists or if the event happened at all (meaning "anyone"). If you used кто-то, it would imply you know or strongly suspect that someone specific unlocked it, and you are just trying to figure out who.

Why is закрыт in its short form here instead of the full form закрытый?

Закрыт is a short-form passive participle. When describing a state that acts as the main predicate of a sentence (like saying something "is closed"), Russian requires this short form. The long form закрытый is used when attaching the word directly to a noun as a modifier, such as in закрытый сейф (a closed safe).

Could I just use открывал instead of отпирал here?