I would guess the vast majority of hidden bbfs are hidden because the client thinks they are getting special consideration and don't want to ruin it. This obviously doesn't apply to providers who are known for this service (kallie, Gina Rollins).
I know that at least once I have had a provider bbfs (spur of the moment, or so it seemed) and then say please if you review me don't put that we did this. What I did was review them without lying or breaking my promise. I went thru the experience and didn't mention putting a glove on. But also didn't say it was bbfs.