I had gone up some stairs and with my M4 up and ready, I went to open door with a single click of the Middle Mouse Button (MMB), but my movement disabled the open door icon and instead the animation went to loading a 30 round mag from my equiped 40 round magazine. In my hastiness, during the reloading animation, I had the open door icon appear and pressed MMB for a single click door opening and somehow during that evolution I had lost my 40 round magazine.