I just had this issue with the sub tank light staying on three days ago. I disconnected the main battery for 10 minutes and the sub tank light stayed off when I started the car the next time.
Thanks for the tip Evil Twin.

As a side question, does the sub tank ever engage automatically, like when the fuel warning light has been on for the main tank for any period? Or is it just triggered manually with the push button switch on the dash?

I filled my sub tank in November 2018 and it has been dropping slowly. The needle is sitting at half full now after 10 months.
I run my main tank to empty before refueling each time and I've filled the main tank exactly 10 times since filling the sub tank.