[jojobas]

They weren't just quickly inspecting it, they found invisible ink on it.

[082349872349872]

Outgoing international mail sent by a nonlocal tourist might have been a quick triage signal for the local postmaster?

These days, of course, our communications pass through various boxes that can automate inspections, so no need to involve wetware.

Keyword search in 1960s US: https://en.wikipedia.org/wiki/IBM_7950_Harvest#Usage

[jojobas]

Nothing makes a "tourist" letter any more likely to be spy-related than any other letter going out of the eastern bloc, say a pen-friend letter to Sweden.

They probably were inspecting all outgoing mail.

[jacquesm]

And as well for all the satellite states.