Update...
I put in new batteries this Monday and at the same time switched back to using my Logitech mouse instead of the Magic Mouse. The batteries in my keyboard now report 92% charge. The last time I changed the batteries with the Magic Mouse connected, after one day the keyboard's charge was down to 72% (or something like that - in the 70's). So, it is really looking like it has something to do with the Magic Mouse (not just having the latest Bluetooth, etc, updates). I won't use the computer much, if at all, over the weekend so it will be interesting to see what the battery state is on Monday.