I thought I'd bring some closure to this with a "reveal."
Disconnecting the "extra red/black" wires solved the problem of the lights turning themselves on, but it was very unsatisfying not knowing where those wires went and what they did. I borrowed a toner generator and inductive amplifier from a friend in an effort to trace these down. Attached is a photo of "culprit:" an X10 switch located in a corner of the courtyard.
The wall switch in the courtyard is only 20 feet away from this X10, but I've got a hunch why it's there. This spot in the courtyard was where the hot tub used to be. I'm guessing this was so you could turn on the courtyard lights while you were in the hot tub. Or, maybe more accurately, turn OFF the courtyard lights! The architect who built and lived in this house for 30 years would have been mid-40's when it was built in 1979. I'm guessing this house has seen some shagadelic hot tub parties in it's time. On a side note (but related to shagadelic): I've got all the original plans for the house, which included a fairly complex solar hot water system. I cracked up when I finally figured out what the pump marked WB was for... that was the solar hot water for the water bed!
I'm totally unfamiliar with how these X10 devices work, but I assume it either has a timer component that started turning on randomly or a short. Thanks to all for the feedback.