Was it below freezing? Sounds like something was keeping vacuum from reaching the sensor, or the brake pump wasn't kicking on. If you can pump the brake pedal fast enough, you can re-create this warning because the brake system runs low on vacuum.
Could easily have been a software glitch since it went away after a restart. Way back when, I had a relay clicking like crazy after starting the car, and was loosely related to if the car was putting out power or regenning. After a restart, it quit, though every once in a while it will still click on then off, but always when starting out from a stop when it does it.
This morning, the thermometer at the house said -7 F, though it felt much colder (never checked what the car said). I noticed the brakes were really stiff, but no warnings. The car also had some trouble locking into Park. The lever went to the P, but struggled to settle into the final position.
"Bear" - 2012 Diamond White Pearl ES with QC - 2/21/2013
Solar-powered since 10/10/2013
"Koorz" - 2012 Cool Silver Metallic ES with QC - 1/5/2015
2017 Bolt EV LT in Orange with QC - 7/31/2017
Driving electric since 2-21-2013.