A Game of Thrones/A Clash of Kings; GRMM

I'm currently rereading the Song of Ice and Fire series, and have just read the part where Catelyn Stark meets with Renly and Stannis. At this point Catelyn is aware that Arya is not being withheld in Kings Landing.

Apparently I've missed where she finds out that this is not the case. I wonder if anyone can help me shed light on this?

via JREF Forum http://forums.randi.org/showthread.php?t=263945&goto=newpost