Sounds like too much moisture. Sage likes hot and dry, so if you've had a wet summer both the rain and the humidity can cause the leaves to rot. Here in Florida, I actually move my sage inside to a south-facing window in the summer.

Kevin - if it was outside you should leave it outside, maybe with a little shelter from a bit of fleece or something held well off the foliage. Bringing it in might well be the cause of your problem. Rays of strange yellow light coming through my windows today. What is it??? Still b..... freezing though.

I have the same thing this year on my sage. I took a leaf to the county extension agent and she knew right away that it's caused by the four lined plant bug. I hadn't seen one, but after it was identified and I knew what it looks like, I started to spot a few. It's active mainly in the spring but it lays eggs in the woody stems, so may come back next year if you don't get rid of them.

Might try looking under the leaves. Neem oil is effective for some problems. It could be fungal too, so look carefully even if you can check at night.