[He feels the tug (although "feel" really isn't the right word for it since that implies tactile sensation, something that's a bit beyond a computer program), and his attention is snapped back to the other presence, the one on the other side of the wall that is currently trying to follow him home, so to speak. This is...bad. Very very bad, and he can't afford to let her finish the trace, so he shifts some of his attention back to trying to cut her off before she can get there, keeping most of his attention on breaking down the next wall.
Unfortunately, with his presence split like that, none can work very effectively or efficiently. The wall fares better, or more accurately worse, since that's where most of his efforts lie, but even so the work isn't as elegant, nor as quick as before, and while he's still trying to exploit the weaknesses he's found in the next barrier it will take him much longer to get through this way.
Heading her off, preventing her from finishing the trace, this has even less effectiveness. He's trying, and he's successful to a degree, but it's like the easiest game of hide and seek ever, a participant hiding behind the curtains or under a chair; he doesn't have enough attention to spare to re-route enough to hide well.]