Keeping 100% situational awareness up 100% of the time is exhausting. Skittles could easy have is focus on an object or some other group of biters only to be surprised by another group that wandered closer to him. He may have then noticed that they did not pay any attention to him. Then, he just has to experiment.

Besides, he already had an inkling ( bahdump bump ) that they ignore him after the Dunbar invasion.