I daresay the vast majority of situations have some level of A involved. One of the hard lessons is that all people are capable of an A in the right circumstances - it's just the circumstances that vary from person to person.
If there's suspicious activity or even a feeling that something is not right - it's safe to operate on the assumption that there's an A until proven otherwise. That's how likely it is. I'm sorry to be so blunt, but it helps to think this way early on.
When I read a situation where the LBS says they are lucky there is no A I tend to think the likelihood of there having been one that was either well-hidden or where the LBS is in denial is pretty high still. This seems to be particularly strong with WAWs, who may have been ready to leave for years but were waiting for the safety net of a new man.
That said, with time and patience, working through it all, you may find that you can get past an A. Once the initial sting wears off, people find that while the "special" aspect of the relationship you mention may seem to be gone, there's still something much more substantial worth fighting for.
It is possible there has been no A and I don't mean to offend you, but given what you've said I'd operate on the assumption that there is at the very least an EA going on.
Me: 24 W: 24 T: 9 M: 6 S7, D4, S2 PA Starts, ILYBINILWY: Nov 2012 BD & PA Discovered: Jan 2013 First ML since BD: April 2013 Physical separation: Mid-May 2013
