I know the general thought here is that you should let the OBS know about an affair in most, if not all, circumstances. That doesn't apply to me because OBS and I found out within a few minutes of each other.
However, I know that my WH and the MOW had at least two occasions (probably more) that they spent with another couple who are both married to other people. I don't have any proof that I could share, but I have friends that are encouraging me to tell the OBSs anyway. Neither of them are friends of mine, I have met them each once or twice and could find out how to contact them if I really wanted to. But, is it my place to say anything? It would be one thing if I had proof, but I am afraid that contacting them without proof wouldn't make a difference and could end up blowing back on me.