I know this is an old post but somebody may be looking for the right answer one day.
There is no electronic sensor on the pads, the only sensors are the metal sensors on the pad backing that scrape the rotor when the pads are too low, but that has nothing to do with the light, direclty, and the ABS module has nothing to do with it either. It's actually a very simple and basic system.
When your pads wear down, the piston in the caliper needs to be extended more to keep the pads right next to the rotor, that's why you can't feel your brakes getting low, like in the old days with drums, unless you still have rear drums, then you'll feel it when they wear, but the pad never has to travel more like drum shoes do when they wear down. In order to keep that piston or pistons extended further out, more brake fluid has to fill the bore or cylinder behind it, which means more fluid is out in the system and not in the master cylinder.
When there is just enough fluid missing from the master cylinder, the sensor in the resevoir detects it and flips the light on. That's why when it first starts the light tends to go on and off. When you step on the brake more fluid goes out into the system and at the same time the fluid in the resevoir swishes forward, both of these things drop the level just enough and trigger the light.
When you let off, fluid returns to the master cylinder and the momentum of the vehicle changes and the float switch opens again, turning the light off.
When you replace the pads, you or whoever does the job, pushes the caliper pistons back in before seating the new pads in the caliper, so all of the fluid behind them goes back through the lines and into the master cylinder again, thus raising the level and keeping the switch open and light off.
This is the main reason you should never add brake fluid, if the level is low, most likely it's out in the brake system and you are going to need brakes. Unless you have a problem it's not going to leak out, it's a sealed system, and if it is leaking you will know it before that light comes on. It's not like the engine oil, that can burn off or slightly leak from a few places, so you need to replace it, you generally will never have a brake fluid leak unless something is very wrong.
If you add fluid, then whoever does your brakes will have an overflow mess when they push your caliper pistons back in, believe me it's happened to me plenty of times and it's a mess, not to mention brake fluid will strip the paint right off your firewall, so it's not a good idea to ever add any.
Even if your fluid level looks OK, it is probably just at the right level to close the switch intermitently and turn you light on.
20% off pad left sounds about right to see an intermitent brake light on.
Hope this helps somebody doing a search at some point.
Later.