Severus touched the window lightly, trying to see into the dark room well enough to tell if Alecto was waking and if the thing was bolted. It was and to unlock he had to get his feet planted on top of the window frame below. He startled as his weight returned with a simple utterance and held tight to the sill, closing his eyes a moment to calm his heart.
When he was re-focused, he took care of the window lock and started pushing it open as quietly as he might, then pulling himself inside, making a bit of noise in the meanwhile but eventually dropping to the floor without any real crashing, which was good because he'd expected way worse. He didn't bother closing the window up again and instead crept over to climb onto the bed and over her, sure that'd do the trick of waking her up.
"Don't scream, it's Severus," he said, realizing that most people would panic if someone came jumping on them in the night. Oops.