I pulled the wiring harness (plug) off the back of the ignition switch and found that the plug does not seat tightly and there is no boot to keep dirt out of the contacts on the back of the switch. There were bits of grass, etc. bridging the hot contact with one of the others - it wasn't a bad enough short to interfere with running but it was enough to drain the battery when it sat. I cleaned it out, replaced the plug and wrapped it with tape to keep out the dirt. So far, so good.