A SPY

No idea of this particular instance, but there is a general idea of a shibboleth as a normative indicator someone is a member of a group by common tradition or custom or something not done by taboo. The way you say a word, react to a sneeze, count on your fingers, pronouncing it sequel vs S-Q-L, etc

https://en.wikipedia.org/wiki/Shibboleth

https://en.wikipedia.org/wiki/List_of_shibboleths

I've always preferred "gesundheit" as it's wishing the person good health rather than using religious terminology.