The rocker who is celebrating the twentieth anniversary of his album Sunsets of Empire this year, revealed his 007 act of bravado happened in Brazil shortly after the album's release.
Following a gig there, he told BBC radio Scotland quiz show Stop The Press that he and the band were having one last bit of chill out time before hopping on the plane to the next destination
He said: "We were all sitting there drinking coffees and were a bit worse for wear. We all got up to move towards the gate and were about thirty metres down the corridor when we realised we'd left the briefcase in the cafe."
"We had the whole tour to go, Argentina, Chile and all the countries after that and all other documents and visas were in the case".
After realising the place was empty, and the briefcase was gone, Fish whose real name is Derek Dick took matters into his own hands.
He said: "There was no sign of the briefcase and two shady guys were working behind the bar. I just knew that they had taken it so I took advantage of the situation. I had been in Bosnia earlier, about a year before, and I had kept this UN identity card that looks really mean. I don't know what came over me but I just pulled out the card and went 'UN security stick your hands up against the wall'. I jumped over the bar and put the two guys against the wall and it was like I was holding a weapon. I kept saying: 'Where's the briefcase. This Is a UN matter and we want to know about the briefcase right now' and they wouldn't talk to me."
Things got heated after Fish realised he wasn't the only undercover agent there.
He laughed: "I turned round and there were three Brazilian FBI officers behind me with real guns.
"When I was in Bosnia I met some guys in the special forces and they kind of told me things to do, little tips, and one of them had said 'Don't say anything'. So this particular FBI guy said what are you doing and who are you working for and I said 'I can't tell you.'I then told him 'These two guys have taken my briefcase' and explained the situation saying 'It's behind there.'
They kept asking who I was and I said 'I can't tell you. ' They walked me all the way round the airport for about fifteen minutes and we went into a room, me and the two guys from behind the bar and another bunch of FBI officers.
I explained the situation and said: "These two guys have taken this briefcase and its in the back of the cafe in a cupboard so just go to the back and you will find it. They took me away and walked me around for another ten minutes saying who are you and 'I said I'm sorry I can't tell you. You've got my passport and I can't say.'"
Eventually Fish was allowed to board the plane but the officers still had no idea who he was. He said: "They took me up to the plane and I was the last person on that plane, walked up by the Brazilian FBI. They turned round and said to me 'We found the briefcase in the back of the cafe' and they shook my hand and said 'Nice working with you'.
The rocker who is threatening to retire next year following his sixtieth birthday says he is now planning a double album to end in a high note .
He said: "I want to bow out with a big statement."
The showbizlion.com appears on BBC Radio Scotland's Stop The Press this Friday.
From Scotland with Love - The Showbiz Lion