Well, it's tomorrow and I’m thankful for your suggestion to test for
leaks with a hose. The exterior slots drained nicely with no water pooling up inside. I discovered a leak entering through a pop rivet that attaches the curtain rod. This may have been the culprit all along. Who’d a thunk it? (Yea, I know. I said there were no sources for leakage above the window.)
Is it reasonable to squirt a temporary filling with latex caulk and try to find the little aspirin-sized covers to put on correctly in the Spring? (I’m not really procrastinating here. I suspect I could do a nicer job on all 20 or so covers when it’s not so cold.)
Tom