‘Mention of this locale does not occur before the story The North-Star,written around 1937-38. This can no doubt be explained by the fact that Simenon didn't actually "discover" it until the 1930s, when Xavier Guichard invited him backstage at the police, as reported by Simenon in texts appearing in 1934 in Paris-Soir. In 1937, he would put out another series of texts, entitled "Police-Secours, or, The New Mysteries of Paris", in which he describes precisely the locale of Police-Secours and the activities taking place there. These two sets of texts have been collected in the volume, "Simenon, my apprenticeship" (Simenon, mes apprentissages)", from Omnibus. Here are some extracts...
"In the large room with the iron door, but two windows open to the night, there are four, four peaceful officers, with two of them wearing gray smocks... On the left, an enormous piece of furniture which resembles a telephone switchboard, in which hundreds of little lamps are ready to light. On the right, a telegraphic device which runs from one moment to the next. Finally, above... we hear the steps of a "solitary", a fifth officer who, he alone, waits before his equipment to send out radio calls. ... Just now a lamp, as big as a lozenge, lights up on the map of Paris attached to the wall. It's the lamp of the 13th arrondissement and its blinking signifies that the Police-Secours car of that arrondissement has just left. ... Already the operator has taken up the telephone which will put him in direct connection with the principal station of the 13th. ... The station there, Place d'Italie, is not yet aware. It's one of their agents who has broken the glass of the call box on the Rue de Tolbiac, thus requesting reinforcements. ... I am once again in this vast room at Police Headquarters where hundreds of bulbs, lit or unlit, are such witnesses of the dramas of Paris. ... I am for the last time at the heart of this network of lines which transcribe to the illuminated table of the Central Bureau all the diverse facts of Paris. It is night. There are five of us in the midst of the apparatus which goes on and off intermittently."
As said above, this department is mentioned in l'Etoile du nord, where it's a call from the officer on duty at Police-Secours which will clinch Maigret's investigation. Police-Secours is mentioned in many novels, sometimes when Maigret receives a call from them, sometimes when he calls them himself to learn if something has happened in Paris. A more precise description of the room is found in three novels. It's how Maigret and the Fortune-Teller (Signé Picpus) opens, at Police-Secours... "Three minutes to five. A white bulb lights up on the immense map of Paris which covers one entire section of wall. A worker sets down his sandwich, inserts a plug into one of a thousand holes of a telephone switchboard." Later, the locale is described as a "room which is like the brain of Police-Secours." It's there that Maigret has come to await the announcement of the possible murder of a fortune-teller…’.
See here for Wenger's fuller discussion of this locale.
See here for representations of this locale in other Simenon adaptations.