When I peeked into the room, nobody was there.

Literal

Room [object-を] peeked [try-てみ] [when-たら], nobody was-not.

Combines ~てみる (try doing, to see what happens) with ~たら (when/upon doing). のぞいてみたら conveys 'I peeked to see what was up, and then...' — the たら introduces the discovered result.