Miss Gardner was part of a small team of examiners who secretly went through mail that came in diplomatic pouches. The team were experts at what they termed ‘chamfering’. They could steam open letters, usually with a little kettle, and reseal them. They could also unwrap packages encased in a web of twine, examine the contents, and then put everything back as though the contents were never disturbed.

http://www.royalgaze...SLAND01/710049995

As mentioned in the reference, USA after entering the war, took over examining all transatlantic mail. Hey, it's not on US soil!

I got to see some pictures of this operation on a cruise to Bermuda.