"Some people used not to bother locking their bikes, until one was stolen. Unfortunately the gates and barriers open automatically when you approach from the inside, so the thief (who was presumably someone with a badge) would have just jumped on the bike and ridden away. The guards at the barrier probably even said "Goodnight" as s/he left..."
Similar situation to my loss. In a Territorial Army centre, I forgot to lock my bike to something secure, though it was locked! The 'boss' was convinced it was someone just playing silly b***ers and hiding the bike. I looked everywhere, in the TA centre and it defnately was not there. As my insurance wouldn't pay out without a Police Crime Ref number, I called the police, who called the Military Police (as it was on MOD property). Nothing came of the 'investigation' as I knew it wouldn't, but it got me my CR number, which got me my payout. After that, the security in that Ministry of Defense establishment got beefed up, as I expect the 'boss' got his rear chewed by his 'boss' for the embarrassment caused.