Fluid reservoir with means to detect the installation of a cap

The reservoir has a filler hole (10) delimited by a neck (11) that has a conductor track (40) with which conductor units of a plug (2) contact when a cap is properly mounted on the neck. The conductor units are situated on a peripheral part of a portion (20) of the plug, where the portion is fitted in an upper part of the neck. The plug is maintained in place by a flange (3) screwed on the neck.