It seems to me that the Basilisk just didn't leave the pipes, except for its head. Staying inside the pipes, it could avoid everything. It went to find it's target, then simply poked it's head out, attacked, and went back in. The portraits might have seen something, but it seems to me their gaze would be more along the hallways. It seems like the attacks all happened near Bathrooms, and there was always a puddle of water around. However the Basilisk made the attacks, it didn't wander much outside of the plumbing, and for whatever reason, the pictures didn't see it.

It seems to me that the Basilisk just didn't leave the pipes, except for its head. Staying inside the pipes, it could avoid everything. It went to find its target, then simply poked its head out, attacked, and went back in. The portraits might have seen something, but it seems to me their gaze would be more along the hallways. It seems like the attacks all happened near Bathrooms, and there was always a puddle of water around. However the Basilisk made the attacks, it didn't wander much outside of the plumbing, and for whatever reason, the pictures didn't see it.

It seems to me that the Basilisk just didn't leave the pipes, except for its head. Staying inside the pipes, it could avoid everything. It went to find it's target, then simply poked it's head out, attacked, and went back in. The portraits might have seen something, but it seems to me their gaze would be more along the hallways, and at least my mental image is that the basilisk came out of the ceiling. Thus, it wouldn't be seen by the portraits.

It seems to me that the Basilisk just didn't leave the pipes, except for its head. Staying inside the pipes, it could avoid everything. It went to find it's target, then simply poked it's head out, attacked, and went back in. The portraits might have seen something, but it seems to me their gaze would be more along the hallways. It seems like the attacks all happened near Bathrooms, and there was always a puddle of water around. However the Basilisk made the attacks, it didn't wander much outside of the plumbing, and for whatever reason, the pictures didn't see it.