I have only ever come across a risk so significant that it warranted a fire controlled letterbox/plate.
It was the HQ of the traffic wardens in a very busy part of London. An anonymous enough looking building of 4 or 5 floors and a single staircase. You would often find 40 to 50 wardens in the building and a trail of uniformed colleagues coming and going. I was not difficult to spot, especially as it was on a main road.
They had suffered vandalism - with a number of unpleasant and largely organic items being smeared on and through the door
As even the most level headed person can lose it with these knights of parking, and with only one means of escape, I suggested they invest in an anti-arson box. They declined as their last arson attack was more than a year ago so disagreed with my assessment of risk!!!
The customer is always tight, I mean right aren't they?