Either that or the maps really had that word printed on them, for whatever reason.

"Секрет" is more a social thing. Confidential information of bigger entities like government or business will be called "тайна".

Of course Russian has the word «тайна». It's the generic term for hidden, classified, protected, secret, or unknown information. And «государственная тайна» would be "state secret".

But the stamp on the maps wasn't just for indicating that they are some sort of state secret. It was for indicating the exact level of secrecy in the Russian classification system.

Those levels are «особой важности» (literally: of special importance), «совершенно секретно» (lit.: completely secret) and «секретно» (lit.: secret). And in US government terminology, they approximately correspond to "top secret", "secret", and "confidential", although the boundaries between the levels are defined somewhat differently.

In Russian Empire the imperial court mostly spoke French so lower classes, aspiring to sound more like nobles, borrowed a whole lot of French words. Because of this many Slavic words now sound archaic or pretensions to a Russian speaker. E.g. тайна in Russian is more like 'enigma', definitely not 'classified/secret'.

> "tajna."

That would be a noun, in other words, a particular state secret. You'd say "Eto gosudarstvenaya tayna" - this is a state secret.

But in English secret is both an adjective/adverb and noun, in other languages there might be two different words for it.

So it would be "secretno"(ru) = "secret"(en) and you might say "Eto sovershenno secretno"(ru) = "This is top secret"(en).