Do you have any quotes or examples that point to her Compulsion being undetectable via Delving?

My impression was that by "subtle", it was meant that the behaviours of her victims are still highly functional and seem perfectly normal. Ramshalan fits this: despite the complex weaves place upon him, he still retained full cognitive abilities, and appeared to be perfectly normal until he was Delved. Compare this to Kerb, or to Graendal's 'pets'.

I must add here that Nynaeve is one of the greatest Healers, so chances are that even if Delving wasn't a surefire way to detect Compulsion, she would be pretty good at it. So good, in fact, that the only way to avoid detection would be to not use Compulsion at all. Even if the Compulsion was very minor and not readily apparent, Rand would have had Nynaeve search incredibly thoroughly for it. So trying to avoid Delving with a subtle weave was pointless anyway. This means that Graendal would either have instantly run, or would have done what she presumably did: use a Compulsion which would be difficult to detect based upon behaviour, and see if she could use Ramshalan to kill Min or something like that.