RSG, sounds like you did pretty well but I caution you to build up what it means, this is a roller coaster ride, the higher up you go the further down the next drop... Don't build this up to mean anything, because it doesn't mean anything.
Doesn't sound like you did, but don't give more information than you need to when talking about what your doing. Try and redirect the question back to her. Did you tell her where your going next Saturday or who with? If not, she will likely bring it up again, do your best to keep it vague.
Also, you don't accidentally end up on someone else's family group on iPhone... Prepare yourself to find out about another man soon... Generally finding out about an A starts with clues, and we choose to explain them away until it hits us head on.. Did she bring it up at all, or was the subject silenced?
I would look for a vet for their opinion on this, but I think I would bring it up and make it clear that although currently separated, you are still married and you will not accept being in or part of an open M, and seeing other people is not acceptable.
M - 9 1/2 years 5/5/16 - Bomb drop - 3 week EA 10/31/16 - We sold house 01/10/18 - D Finalized