As you suggest in your post, I would assume that this may be normal behavior in light of having a rain sensor. The ones I've seen have absorbent discs which hold onto rain water for up to several days (depending on humidity and how much sunlight hits the sensor) until they dry out. This is deliberate, since they are designed to mimic your lawn in terms of how much water is in the ground.
On my Toro digital controller, there is a raindrop icon with a slash through it which comes on whenever the rain sensor is overriding the controller.
You can probably look up your controller's owner's manual on the web and check this out.