There was a small part in the back of Sherlock's brain that heard John come back in, and he knew that he was supposed to say something, but the rest of his mind was so preoccupied with what he was doing, that it was immediately overruled, and he continued The Work.

It was hours later, already getting into the late afternoon when he finally completed his experiments. He wrote the last of his results down, and leaned back in the chair, a satisfied sigh escaping his lips.

That's when the rest of his brain kicked in again.

He got up fast enough to toss the chair to the floor.

"John?" He looked around desperately for the man.