Which type of Hunter rain sensor do you have? Can you post the model number? I think it has to do with either the way this particular sensor works, or there may be a loose connection somewhere..
I am 99% sure it’s the Hutner Mini-Clik, but I will confirm when I get home. Here it is:
(product page) http://www.hunterindustries.com/irrigation-product/sensors/mini-clik
If it’s a simple normally closed sensor, when it detects rain the two wires on the rain sensor should remain open-circuit. You can use a multimeter to verify this. Normally open sensors are the reverse way. If the multimeter shows that it connects and disconnects and repeats like that, then the sensor will not work with OpenSprinkler.
Everything I read suggests that it is a simple normally closed sensor (there is a 3rd wire to provide support for normally open controllers, but this isn’t used in my installation). I confirmed with my multimeter that it showed “open circuit” when the sensor itself was wet. I did not measure the wires while they were attached to the controller, but rather disconnected them and then measured at the tips of the disconnected wires. It was not inconsistent — I had a solid/stable “open circuit” reading. I did not test the sensor with a meter when it was dry, but I will do that this evening (that said: the “no rain” status has been working correctly, the problem occurs only when the sensor gets wet).