In my experience, this is quite a common thing to happen to multillingual people when living, or just out on vacations, abroad.

I lived 2 decades abroad and this kind of thing happenned to me a couple of times.

Mind you, once you trully master a foreign language you start being able to tell accents apart, so are more likelly to spot that somebody is speaking that language with an accent from somewhere else, but it's pretty hard and takes time to reach that level of mastery of a foreign language (personally I only ever got there with the English language) so it's more likelly one is just good enough at it or even fluent but can't spot that, say, the person you've been speaking to in a foreign language is one of your countrymen.