"Hey, that's weird," said Fred, frowning down at the Map as he leaned against the wall. George was a few feet from him, scanning the wall for the entrance to a secret passage the Map indicated would lead down several floors from the Defense classroom down to Transfiguration, which would save them a walk on Tuesdays and Thursdays. More importantly, it would confuse and impress the rest of their classmates, which was of course the point.
George looked up from where he was examining a wall sconce under wandlight. "Somebody awake?" he inquired curiously. It was past midnight, and Fred's keeping an eye on the Map in case of interruption was mostly a useful habit. They hadn't been expecting to run into anyone; Filch was out on the grounds delivering Ron and Malfoy to detention with Hagrid (maybe Ron wasn't a lost cause after all), and periodically checking the seventh floor showed that Mrs. Norris was skulking around Ravenclaw Tower. The Map didn't show everything all at the same time, but it showed you whatever you thought was important, which was just as good if you had a good sense of priorities.
"Apparently," said Fred, "It's - Quirrell, I think - just look." He pointed at the Map with his lit wand, and George paced over and looked, too. Fred had been thus far not worried about Quirrell, who was pacing around his office down the hall but showed no signs of leaving. But a moment ago, Quirrell's name had started flickering, and going all blurry, and then it showed "Tom Riddle" instead, and then blurred out and went back to "Quirinus Quirrell," and then changed again, flickering quicker and quicker.
George didn't bother to say anything affirmative, like 'Oh, yeah, that is weird,' or 'Nice catch'; there was no point. It was simply a brute fact of the universe that they had the same opinions about everything, that they tended to think the exact same thoughts when given the same information. The only time they ever needed to talk to each other was if, like a few moments earlier, they were observing different things. (This was why, perhaps, it had been traditional to kill one of a pair of magical twins even as late as the previous century.) Instead George pulled a scrap of parchment from his pocket, and a quill that thankfully still had some ink, and scratched onto it Tom Riddle, so that it could be looked up later.
As they watched the name settled, and then Tom Riddle stood stock-still in the middle of his office for a long second - and then he dove out the window. Fred and George made a collective and involuntary gasping sound of shock; they were on the fifth floor, and it was a very long drop to the ground. The Defense professor did not apparently seem to mind this obvious problem; as the Map followed him across the grounds, he sped across it at a rather alarming speed, and disappeared into the Forbidden Forest.
The Weasley twins concluded that he probably had a broomstick. Not being able to see unanchored inanimate objects like that was one of the Map's limitations, after all. Although there wasn't any sensible reason for a teacher to have a broomstick just lying around in their office, there was no other way to explain his apparent failure to experience injury from falling five stories. For that matter Professor Quirrell seemed like the type to be terrified of broomstick riding, but it wasn't like he was immune to gravity or something ...
Then they spotted Mrs. Norris coming down the steps, and bolted in a hurry, saving the mystery for later.