the balloon goes up

[Ivo]

Would you tell me whether I am right with my interpretation of the expression in bold in the following sentence?

A chap of ours in Denmark in 1940 saw from his own window the German fleet coming down the Kattegat… Of course he knew then the balloon had gone up. (Gr. Green, “Our Man in Havana”)

the balloon goes up = something has begun, usually something very unpleasant

Thanks for your efforts.

[Beeesneees]

You have obviously looked up this phrase.
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Cheers m' dears!