The BBC reports suggests that the Captain first aroused the interest of security personnel when liquids were detected in his baggage, presumably at the staff security checkpoint. The suspicion that he was under the influence must have come to light in the ensuing conversation. Perhaps this serves as a reminder to all crew that chronic alcohol abusers are secret addicts and are well practiced at hiding their addiction.
Having met the crew in the hotel lobby over a coffee, and travelled with them in the confines of the crew transport, it seems strange that nobody noticed anything out of the ordinary. What I find most scary is that he attempted to take his alcohol on board, presumably to 'top-up' during flight.