I remember a Benny Hill episode taking place in a Victorian-era nightclub, and a Prussian military officer in a pickelhaube is shown being escorted to a table, the joke being his having loaded pistols which keep going off (but only causing flashing and noise).

Okay, maybe a trivial question, but if a Prussian officer were to go to a nightclub in a foreign land, not part of an official ceremony, how would he actually have been dressed?

Hello.
This would be an example. Official ceremony or not. An Officer in that century was a class of its own and always stands for its class in full designated dress for special occasions.
Like this:

A foreign officer would immediately recognize his opponent to mingle with.

Kind regards,

Robert Jan

Once upon a time when a gentleman officer was out of the barracks he usually dressed in mufti.

A lot of the culture is more accurately represented in the 1968 Charge of the Light Brigade

The profession of arms was always looked down upon so many officers didn't want to advertise it. But in their own circles they'd wear their uniforms. But remember partying is hard on such delicate clothing as a full dress uniform. It's made for parades not a night out on the town.

There was a off duty "town dress" in some armies but again soldiers didn't want to broadcast their low standing in society when socializing.

As for the skit. His pistol goes off whenever a pretty girl goes by? That's right out of Shakespeare. Falstaff and Ancient Pistol.

In the actual skit, the gun went off randomly, but for comical effect (such as causing a waiter to drop a bunch of dishes).

I was wondering why a military officer couldn't just wear civvies if he's in a foreign land and not momentarily involved in a conference or other official function.