Thanks.
The score multiplier is not meant to be secret. I’ve just not had time to document it yet.
Every time you destroy a robot, your score multiplier is increased by one. If you do not destroy another robot within one second, your multiplier is reset back to one. So, you have to destroy robots in quick succession to keep building your score multiplier.
If your score multiplier is greater than one, the HUD will show a blue indicator light (well, single pixel – the game has 64x64 resolution) in the top-left corner of the screen above your lives indicator.
The game notifies you by a message in particles and by sound effect when your multiplier reaches 10x, 25x and 100x.