Several of you had suggested I checked the pipes for leaks. I did. Twice. No visible leaks.
Two days the water supply to the boiler was running. I was running out of ideas when it snowed again. While out shoveling that mess, I heard running water, like a brook. Looking around, I find a brook running out of my house!
Back to the basement. No water anywhere. I finally found two pipes leading out under the front porch. Why!? There's no radiators out there. Um, whoops. Yes there is.
I cut the feeds and capped them. Suddenly the pressure builds at the boiler. Hurray! At five psi, the relief valve pops. Thump. <that was me> Water everywhere. Agh!
I run to the plumbing supply house, get a new relief valve. And away we go. I might even light the burner ... but I'm afraid to.