That is, maps printed on the inside of the envelope. I’ve received one in the office from France, is this a security or recycling device? Or something unrelated?
I’ve seen envelopes like the ones you describe that are made from recycled paper. They weren’t from France, though. I saw them in the US.
Here’s one company making maps from recycled maps, there are others.
Interesting, there’s a recycling mark on the French envelope, but it was whisked away to the confidential waste before I could note it down.