Aprilaire 500 not turning off

I have an Aprilaire 500 with the digital humidistat, outdoor sensor, and independent power source, hooked up to my Goodman furnace, humidifier independent blower activation is set to off. All has been well since install 2 months ago. About 2 weeks ago the wife heard water running. Looked at it and while fan and furnace were off, water was flowing. Turned humidistat off and it stopped. Turn back on and it started again. Heard the solenoid open/ close. Turned furnace system off with power switch and back on. All worked fine for two weeks and now same issue tonight, "rebooted" furnace with power cut and now fine. Note when I cut power to furnace I did not cut power to humidistat. It's like it thinks the furnace is still on. Any ideas? I have triple checked wiring and all looks correct. Thanks