I wonder if she set her response to you not staying? She could have had the same mindset you did, except maybe staying over was her baseline. Its possible that in her mind she figured if it ended in a night of passion that was her sign, and if you left that was a different sign.
Together:20 years M:3 years Me:40 WW:40 S15 A suspected:5/17 AC:5/18 BD:8/18 WW in full blown R w/ OM Still under same roof