Gordie - I'm being "Mr Cold Bucket of Water" these days - so sorry in advance.
For quite a while before she moved out I'd thought that my ex had broken up with OM. She was (mostly) transparent about her goings on, we talked a bit - but not R talks etc. Somebody posted to me the very wise comment that his own wife had also been very transparent - except when she wasn't. A few weeks before she finally moved out my ex started "going out with a friend for dinner" on Friday nights. I suspected it was with OM but didn't know and like you didn't ask.
There's a concept called "branch swinging" where people will hold on to their existing partner until they have a solid grip on a new one. I believe that my ex did that even for 6 months plus after she moved out by keeping everything quiet and bolstered by meeting me once when presumably she and OM were on hiatus and talking about how so very lonely she was. Not talking about coming back but she seemed very happy when it was obvious that I was still a fish firmly on the hook. What she didn't know was how precarious that was and on the first indication that she preferred OM to me I threw in the towel which I suspect was quite a shock to her.
Just my 2 cents.
On BD H52, W50 T27, M26 S21, D23 BD-9-Mar-16 D-15-Jan-18 Final-19-Apr-18 I am a storyteller. The story may do you no good. But a story is never for the listener. It is always for the one who tells