I found an example. This “polyamory” blog discusses this very issue. Polyamory is the term used to describe a situation in which people have more than one lover with the full knowledge and approval of all involved. Okay, well I’d argue that God is involved, definitely knows, and doesn’t approve. But nobody else is forced to believe as I do.
Here is the situation. . . Lucy who is married to Rickie was out on a lunch date with her SO, Fred, when a co-worker who doesn't know about her poly lifestyle saw them together. Now seeing Lucy and Fred together wouldn't have been hard to explain except the two did a bit of hand-holding at lunch.The blogger goes on to consider the possible approaches of the observer and how to respond to those approaches. So even if we think of this as infidelity, we can't assume that the spouse who isn't present is unaware or would be upset- rather, in thise case, the spouse might be flustered that they have been "caught" as being a polyamorist.
The question is; What do you do when you are caught out on a date with your SO by someone who knows your primary partner (spouse)?