Fire sources or gas impurities are detected and located in one or several monitoring chambers by a device having a main detector for detecting a fire characteristic value or a gas impurity, into which a part of the ambient air in the monitoring chambers is continuously injected by an intake unit through a line, arranged in each monitoring chamber and provided with intake ports. The advantages of gas inlet systems, such as an active intake and a concealed assembly, is therefore combined with the advantage of the localization of each intake port as well as the detection of a precise fire source or a precise gas impurity. To this end, the device is provided with a sub-detector, arranged on or in the area of at least one intake port per monitoring chamber, for detecting a fire characteristic value or a gas impurity. The sub-detector is capable being switched on by a switch-on signal transmitted by a controller according to a detection signal delivered by the main detector.