I carry an "EpiPen" this is a syringe loaded with adrenaline, as I can suffer severe allergic reactions.

I always mention it at check in, once at LHR/T4 on a BA to the US they marked my boarding card and pax record (this was just post 9/11) and when I got on board the CSD came and said hello and asked what the syringe was for. At all other check ins I have just got an "OK", once or twice asked if I have a doctors letter, never asked to produce it.

Always, every darn time, I mention to the scanner team, that there is a sealed medical syringe in my carry on, just in case an over zealous operator did spot something and dive in to the container. Around the world reactions have varied from, "No Problem" to "Don't worry it won't show up on our scanner.." (that was in ORD.)

No one, in I can't count how many flights, on all continents, since I have been carrying the syringe has asked to see it, or my doctors letter.

I too wonder if there is more to this that meets the eye, maybe the pax suffered sense of humour failure and they refused carriage.