Same idea as Mitch, almost certainly due to low fluid level. If not it could be that the float switch in the Reservoir is sticky, just clean this up with WD 40 or a light oil, make sure the lubricant is cleaned off before replacing it in the Reservoir.
I have had an intermittent Handbrake Switch in the past, the indicator light wouldn't always come on when the Handbrake was applied. A few shots of Switch Cleaner and exercising the left arm a bit sorted this one out.