Has Fawn given you the subbe collar? She won't unless you are wearing that.
If you know about her secret life, you must have tried to catch her at the BDSM club and asked if Fawn works at the club... Did you ask Fawn EVERYTHING about the lifestyle? When she started, why, has subbed before, etc.? You have to ask her all the questions about it, then a new one should be added, ask to be her subbe. (Sorry, I've done it like 500 times so I SHOULD be able to tell others "have you done X Y and Z", but it's kinda like muscle memory foe me and when someone asks I have to stop and think...)