Great idea babe, when you head back to the Master suite, perhaps put some pillows under a blanket so when she awakes she thinks you are still there
Slightly different to the 'trying to fool your parents that you are home in bed routine when you'd snuck out for the night' but it might work a treat!!

PS. The pillows under the quilt trick also works VERY well when you go to bed before your husband and then sneak into the WIR and bump out and scare the bejesus out of him when he comes to bed thinking you are snoring away **Evil Laugh**