We were playing twilight, and it was getting pretty dark when we got to the 18th tee. I didn't see my tee shot at all, but I found it in the fairway. If I had not found it, I probably would have just stopped there and posted with my "most likely score." But I found it, and could see the green (sort of). So I hit my approach shot, and I could see it going left of the green (which is where I was aiming). When I got the green, I could not find the ball. It was pretty dark at this point. I was pretty certain that it was not in a hazard. I searched for ~3 minutes and did not find it. So what's the rule?
The only thing I could find in the rules about darkness is when "the committee" decides that play should be suspended due to darkness. In that scenario, if you have started the hole, you have the option to complete it, or you can mark your ball and resume when it's light. In this scenario, I don't think it's unreasonable to call myself a "committee of one." I'm quite confident that my ball was somewhere left of the green. So I believe that my most likely score at this point is a bogey (chip on, two-putt). So that's what I'm going to post.