I'm not sure if this is germane to the topic, but when researching carriage keys, I came across several mentions of security staff confiscating them at the UK end of the Eurostar, which apparently involves a checkpoint (in Paris you just walk onto the train).
They appear to be quite protective of them (although in the words of one person, "anything secured by a square key is not secured").
They appear to be quite protective of them (although in the words of one person, "anything secured by a square key is not secured").