With the Company of the Fox merged into the Gendarme, even with things up in the air regarding the future of Port itself, I find myself thinking this would be a good idea to copy the current garda model of the 'horn' system for the local Gendarme force.
The system could be as simple as giving them a loud bell to ring, or a whistle (being more appropriate than a horn thematically). It could be more complex, working similar to the 1900's system of using a nightstick on a lamp post to create the ringing that could possibly give a more precise location. Whereas the whistle/bell system would say something like 'You hear a gendarme's whistle sound in Quartier Publique' the more precise method would say 'You hear a gendarme's bell call at Quartier Publique near the Governors Hotel.' The first system basically reworks the garda horn and slightly changes the message. The other system would require turning lamp posts into useable items that can be used by a use-specific item that gendarme would have. That, however, would put it in the hands of PC's and not NPC's, so the first might be better.
Understandably, things are a little different in Port over Vallaki, but with that said - the Gendarme are 'supposed' to be a more organised and efficient policing force, so it would make sense to have some manner of alert and communication over distances. This could open up new avenues as well for the gendarme and the pursuit of justice and crime.