Motion-less Detector?

I know I haven’t included any photos yet of myself standing next to Japanese people but an incident just happened that might give you an idea of Japanese people’s stature.
Occasionally during the day I'll take a short break to step outside for some fresh air. The entrance to our office building is a large double glass door with a motion sensor at the top. As you approach the doors, they open automatically. So you can imagine my surprise when as I was walking to the exit that at the same time I could see a Japanese man standing right outside the doors but they weren’t opening. The guy was in early twenties and was dressed in the standard issue black suit. He was probably about 5’5” but had the typically Japanese slight build. Before I got to the exit, the man gave up and started walking along the side to find another entrance. I stepped outside. The man heard the door open and rushed back but the doors closed before he got there. I was off to the side and watched as he stood right beneath the motion sensor but those doors wouldn’t open! Which meant one of two things, either he was a vampire and just like a mirror can’t display the vampire’s reflection, the motion sensor couldn’t see him or he was so slight in stature the sensor couldn’t see him. I contemplated staking him but given that it was 3:00 in the afternoon and the sun was shining I figured maybe I should go with the latter. He gave up trying to get into the building and started walking back towards the road. I walked over to the doors and “Open Sesame” they parted like the Red Sea. I yelled a “Sumimasen” (Excuse Me) to get the man’s attention and when he turned I pointed to the open doors. I had to stand there until he got into the building. Bizarre. I’ve seen those doors open when a bee flew by.