I assumed that he didn't sense it because Demandred was denied access to it. If both Forsaken were granted access to it, I had assumed they'd see the weaves. Does it specifically say somewhere that it can't be sensed under any circumstances by someone other than the wielder? How did Lanfear and co. detect it the first time before the bore was drilled?

I wonder if one of Cadsuane's or Nyneave's ter'angreal with an unknown function can detect it?