They used to have to report anything unusal to thier supervisor. It could be said that a pilot smelling of drink is unusal.
Security staff don't call Police direct, they, as I said report to thier supervisor, who reports things to the next level up. At some stage the information arrives at BAA's control Centre. It is they that call Police, based on if they think Police need to be informed, irrespective of whether or not Police were asked for initialy.
In this case someone took the decision to inform the airlines handling agent as well, so it could be said that the security guard did do his job properly, that the BAA did try to help the guy out. The Pilot didn't take any action as the result of the warning that Police were or would be informed (albeit because he didn't see any need) and as a result he was arrested.