She hadn't asked questions when Buffy had arrived with an unconscious Tim and asked Clary to trap him with runes. She had just done it. She had lined the closet with her symbols, burned into the floor and walls to keep her classmate from going anywhere. She knew Tim had crossed the line, so if they were just going to trap him, she'd do it. When the door had closed she had used her Stele to lock him in, and without her, no one was getting back in.
So when Oliver had arrived and wanted in, she had grabbed her tool and followed. She knew he'd been shot, but he was looking pretty good for someone who had been shot. Yay for healers. She had multiple opinions on this, but she kept it to herself. Was Tim over his run in with Kryptonite? Hopefully. Clary had lost hers a few days in, though it had been fun while it lasted, to be able to jump as high or run as fast as Jace did.
When they reached the closet that Tim was in, she took out her stele and began modifying the runes on the door to open it. Though the door was open, the marks on the floor, all attaching to form a type of trap that lined the whole closet, would still keep Oliver and Clary out, and Time in. She'd have to modify the floor runes for anyone to move in and out.
When the door was able to open, Clary straightened out and twisted the handle, keeping her Stele firm in her hand. She pulled it open wide to let the light in, and then stepped aside to let Oliver take up the door frame, though she did peek around to see what kind of shape Tim was in. "If you want to go in, I have to changes the runes there to." she told him, pointing to the burned symbols in the floor the surrounded Tim.