"Haven't gone out of town yet. I checked the disconnected battery
today 24 hours after yesterday. Now the drain is 4V! Why would it
vary (just sitting there)? The battery is still 10V potential of
course"

What you should be measuring is the amps that are being drawn by
inserting an amp meter in series with the battery. Inserting a
voltmeter like you are doing won't tell you what the actual drain is,
only that there is one. And the drain is likely varying because a
short somewhere isn't going to be perfect. It can vary from a few
microamps to 100amps depending on what's going on, like humididty,
temp, if the short is rubbing against something, etc. One thing for
sure, if it reaches the 100 amp point, you'll likely find it real
quick. BTW, I wouldn't leave the mower in my garage with a likely
short and the battery connected.