Can a woman monkey branch subconsciously?

Myself(38M) and my stbxw(32F) are still living together plutonicly. Things for the most part are ok in the sense that there is tension but no overt yelling or screaming. She has told me after 10 years she no longer wants to be married. I do know we have had problems for years that led us to this moment(which I won't discuss here), but the following seems quite quincidental. She has developed a friendship with another man who she says is just that a friend. It makes me very uncomfortable and I have told her that. She is the type of person that doesn't even admit something is happening to herself let alone anyone else.

I am ashamed to say that I have done some sleuthing but have seen no signs of true cheating. She seems to have kept him at arm's length and truly is treating him as just a friend. But that does not still mean she may "subconsciously" be lining him up for a possible relationship in the future.

My question is this does anyone have experience with an ex that has monkey branched in this way. Or am I just being crazy because I am so insecure about the fact that we really are coming to an end?