>> "Last year MI5 uncovered intelligence which suggested that Islamic terrorist suspects had carried out reconnaissance of the huge Bacton complex of gas terminals on the Norfolk coast. The threat led to the deployment of armed guards around the plant."
> So there were no guards before?!?!?!
Believe it. Rest assured that in your town, there are heaping gunks of terrorist targets which are guarded, if at all, by an alarm system that causes an answering service to place a message on someone's work voicemail.
Generally, guards are placed to prevent crime. Stealing propane by the truckload is problematic. Stealing chlorine by the ton, equally so. Trains don't move unless you're knowing what you're doing -- and pretty much by definition, former train crew would frown on stealing a locomotive. Joyriding, maybe, but neither stealing nor endangering public safety.
Common criminals and crime syndicates, even the murderous sort, frown on large numbers of dead people. Draws too much attention, very bad for business.
So rest assured that someone who knew what they were doing, with materials that you can buy from any garden supply or hardware store, can kill hundreds if not thousands of people through adroit application of basic chemistry, physics and utter ruthlessness.
Any time they want. An armed guard more or less isn't going to make much difference.
The good news is that they don't. The kind of damaged, broken people who would wantonly kill large numbers of innocent people typically fail somewhere between acquire materials and plant device, mainly by talking to the wrong person about it.
The other 99.99999 percent of humanity is getting sick and tired of being told by our governments that we can't be trusted because of the 0.00001 percenters.
As for data centers, yep, they're a bit vulnerable. But I flat guarantee they're Roach Motels for a dedicated intruder. By the time you've done enough damage that it can't be easily fixed, the SWAT team is gearing up in the parking lot.