As Airclues rightly says it was a customs check. they used a device on hands which detects whether you have come into contact with drugs in the las x hours. If it was positive then your name was logged(not passed to the company). There were some positive tests although if you've handled a US banknote for example that can show tarces of coke etc. No drugs were found in any of the random checks. Still I suppose HM customs at LGW have to work once a year.
BTSM