Oh ok. See, I actually tried untying her top, and she called me an asshole and then run away. I was expecting to find it on the ground, searched for a bit, didnt find anything. Then I reloaded, because I didnt want to lose points with Ashley for no good reason.
Yeah, you do have to wait a bit, because I've already mentioned that Katherine will approach you and ask you if you've lost the key, or something along that line. But make sure you do have some kind of relationship with Katherine first.
As a side note: Untying her bra unlocks a new quest, Redemption, if I remember correctly, that can be done easily by collecting the flower outdoors in that little garden patch, then you have to go to the master bedroom's bathroom and use that blue gel to color the flower blue, and then give it to Ashley to complete the quest.