Arc Mouse Mystery

I've got a Logitech MX Revolution and it doesn't produce any visible light* either, even in a pitch black room. I think it also had a warning somewhere not to look directly into the emitter but I don't know how serious that is.

(*Except for the battery meter on the top, before anyone gets pedantic. )

My phone's camera can't see anything coming out of my mouse (but does see other IR emitters, e.g. the Wii bar).

How can it detect movement to know when to turn the light on without the light being on to detect the movement?

...

IT'S A WITCH! BURN IT!

I don't think it's an accelerometer in the MX Revo at least. If I hold the mouse still but move my finger (on my other hand) under the sensor, but without touching it, the battery meter lights up to indicate it's become active again.

I tried again with my camera and I can just see a purple glow coming from the emitter if I point the camera at the right angle.

Maybe it goes into a low-power mode where it takes a snapshot often enough to see if it's moving but not often enough to track accurately, then it goes into full-power mode and takes lots of snapshots, making the accumulated light the camera sees greater.

I still have suspicions of witchcraft and have begun running a bath to do the ultimate test.

It's got a laser since you can see it in the video above. Only question is how it knows when to turn the laser on. My guess it is always pulsing and just pulses far less often when in low-power "am I moving yet?" mode, so it's invisible to most cameras until it starts tracking movement and pulsing the laser more frequently.