For what is worth, it is really impressive how much grip these bikes have. I had never activated my ABS during normal riding and decided I needed to just so I would know what it felt like if I were ever in a panic situation.
I checked it out on good asphalt, with no gravel and absolutely mashed the brakes at 15-20mph. It stopped so quick I either didn't activate or didn't notice the ABS. I tried a few more times with increasing speed and finally noticeably activated them after the 3rd or 4th try. Very non-intrusive in my opinion, I remember just slight pulsing on the handle. Nothing like an early 2000s car that sounded like a machine gun going off and vibrated your foot off the pedal. I don't recall if the ABS light ever actually turned on (of course I wasn't watching the dash).
@dduelin summed it up in the post above well. An ABS system just has magnetic pick up on each wheel that causes a pulse from the notched rings, that is why your ABS light goes off after a few mph as the ABS system "sees" wheel movement. If either wheel does not match speed during braking, it assumes a skid and applies ABS to the entire braking system to make sure that wheel is no longer skidding. On a vehicle with a failed system the ABS light will never go out since it never senses wheel movement to complete the self diagnostics.
When you have an intermittent sensor failure, you will basically have ABS every time you apply the brakes since it is not reading movement correctly, until eventually the system goes into failure mode and illuminates the ABS light and defaults to non-ABS braking. On traction control vehicles this intermittent fault becomes very dangerous as it also limits power when throttle is applied because it assumes wheel spin.
Going a little off course here, but the systems are of course ever-evolving. My 2015 Miata had the traction control system that could be turned off with 1 button push and allow wheel spin, however if it also sensed side motion g-force and if you did not have the throttle during a slide, it would instantly re-engage the traction system and control/stop the slide. You could defeat the entire system by holding the traction off button for 3-4 seconds, and it would cause a system fault, you would of course lose ABS, traction/stability control and run completely manual until you turned the car off and back on for a system start up.
All this to say, if the system has done its self check and thinks it is good, and you are still able to lock up the rear wheel, the only thing that could be wrong is the rear brake taillight switch is not working as
@jfheath mentioned. The system needs to know you are applying brakes before it will activate. In theory the ABS pump could not be working too, but that would be an internal fault and should keep the ABS light illuminated.
Ryan