i thought that too but it seems much simpler for her to ask the messenger where he gated in. then invert her weaves so nobody can sense her. moreover, she doesnt need to gate over there very close. she could have been watching the messenger's return to Rand. she could have been expecting BF given what we see early in the book:

-moridin shows her some favor/reward
-she may have been involved when semmy was sprung
-by telling rand about how BF makes death final, he may have intended rand to use the TP to BF Messy
-its not a big leap for Moridin to expect dark rand to go after the nearest forsaken--Graendal.
-with graendal in moridin's good graces, doesnt make sense she was kept in the dark b/c it would help the dark to lure rand into BF'ing Graendal's palace and tricking him into thinking she was dead. as of this book, we see her value as a tool for Moridin, so why wouldnt he see her value as well? this weighs against the idea that moridin would like rand to off her too just to trim the forsaken fat.

simply put i think its simple and plausible that Graendal was watching and unwove the weave by inverting it and it was not sensed.

moreover, i think the examination/interrogation of the messenger before the bfing was done poorly and rand may have missed an opportunity to catch something