The bank branch (not a datacentre as far as I can tell) had some workers in, presumably out of hours. Those workers, may well have had access to the small server room that they had, most branches don't have many servers, so likely it was a cupboard with a swipy card lock, if the workers need access, this door may well have been set to unlocked. All it would take was someone to hoik the server out of the rack. Bear in mind that if people are working in a branch, they are going to be "trusted".
At the main datacentre/mission controll they would have seen an alert saying the server had just disappeared from the network. At this point you start investigating what has happened, try to connect to it's RIB board, or whatever else it has. This has already taken a couple of minutes, by which time the server was in a sack of stuff taken out to the workers' van and it's off.
Now, that may not be what happened and likely there should have been better security, but it's not hard to end up in this situation, if you trust your staff/contractors.