[Aerkuld]

Here's the situation.
I take the dogs out into our backyard last night around 9pm. The back of our house looks out over the SF bay, high in the hills, with Oakland Airport roughly 7 miles ENE of us. Last night we had excellent visibility, no cloud.
I could hear a helicopter approaching from the direction of the bay heading NW which would likely have crossed the approach to the main runway for the south field of Oakland (runway 12/30?). It would definitely cross the approach to the north field runways. I could hear it approach, heard it fly over, and could hear it fly off to the NW, but I couldn't see it. Nothing.
I would be very difficult to see any aircraft at night without lights. I suppose it's possible I didn't see what was plainly there, but I watch aircraft a lot here and can pick out their position pretty reliably. With this one, even scanning the sky in front of and around the likely location, I couldn't see anything. Not a biggie, but it got me thinking -
Would an aircraft be allowed to fly in this area with no navigation lights? Under what circumstances is this allowed?
Just curious.