He had been nodding off. Waiting in a mostly-empty cafeteria after being on the edge of your seat all day was tiring, and not sleeping properly for the past couple weeks didn't help one stay awake, despite his old lessons. CONSTANT VIGILANCE all you want, at some point, the body gets tired.
Harry had been resting a cheek in his palm and his eyes getting progressively smaller when someone walked past. The small breeze they picked up as well as the note bumping into the hand that rested on the table drew his attention back to the present, and Harry shook his head, peered at his watch for the time and looked ahead at the man who had just walked past. It was Percy. Careful not to frown or to look to put-out (because surely Percy had his reasons for simply walking past - unless it was simply because he had heard something about Harry and simply didn't want to associate with him anymore), Harry looked down at the note which had awoken him.
He waited a few moments before folding it up and putting it in his pocket, and then waited a moment more, periodically looking back at the door as though still waiting for someone. He was probably putting too much energy into thwarting potential spies that were probably not there at all, but Harry didn't really care. At this point, and considering Percy's note, he probably wasn't wrong at all to take all of these precautions. Over the next five minutes, Harry frequently looked over to the door and back to his watch. Percy was long gone by the time he finally rose, walked over to the cashier and bought a cup of coffee of his own. Sipping it, he made his way out the door, to the atrium and to the apparition point, from which he apparated home and then to Percy's, just in case they were monitoring that as well.
Once there, he knocked on the door, but didn't look over his shoulder to make sure he wasn't watched. If someone were there, it would only look guilty.