The quote is saying that noone would have felt Moghedien coming against Egwene in secret because of how Moghedien would have come to get her revenge; even worse, with all of the channeling going on in the camp, she could have come in with nothing but a disguise on her face and hiding her ability to channel, taken Egwene by surprise and escaped via a Gateway.

Keep in mind that this is what happened to Rand with Semirhage and if all went to plan, he would have been in the situation Egwene was hinting at. Nynaeve herself felt this when confronting Moghedien for the first time, and if any non-channeler walked in, all they would have seen were two women staring at each other.

Basically, none of the main characters would like to be helpless near a Forsaken, and the Forsaken are skilled enough to hide themselves until the right moment and none of the main characters might even know until it was too late.