This CO monitor is nice in that it has a readout of CO level in parts per million (PPM) instead of only alarming after CO is detected for the requisite time per UL standard 2034, so if you are curious you can look at the display and see if even trace amounts of CO are indicated (except that display indicates "0" until at least 30PPM have been detected). Also, if you press and hold the Peak Level button, the display will indicate the highest CO level that has been detected since batteries were last changed or the unit was last reset using the Test/Rest button. When you first put batteries in, the unit alarms for about a second, and it seems dang loud to me (of course I'm holding it in my hands at that point rather than standing across the room or in another room). When the batteries need to be changed, the unit indicates "Lb" in the display and chirps every 60 seconds (like a smoke detector). The unit can either be mounted on the wall or placed on a flat surface (I'm using it that way on my desk at work). One reviewer said that this CO monitor does not have a display; I'm quite certain that that is incorrect. Perhaps an incorrect CO monitor was shipped to that reviewer. The unit I have is Model Number KN-COPP-B-LPM (Part Number 900-0146-LP) - this information is taken from a sticker inside the battery compartment. Note, though, that the packaging for my CO monitor omits the "-B" from the middle of the Model Number. See pictures attached to this review showing the CO monitor unit and the label inside the battery compartment.