Who done it??
In 2007 I purchased the “used” 2006 KIA Sportage. It already had 39,000 miles (well used). Eleven years later, the odometer reads 82,131. This year (2018), I’ve driven 1,064 miles.
Last night, a neighbor knocked on my door to tell me the lights were on–on the KIA. Had I been “out” and forgot to turn off the lights? No, I haven’t driven the car for twenty days! 
The headlights were not “on” but the tail lights were bright red in the dark night!! I rushed for the car keys and opened the door. “Yes,” light’s switch was “on” but this has never happened in eleven years!! “Yes,” the battery is dead!!
Did a squirrel get under the hood and chew on some wires? Every day, I see squirrels in the carport. (Especially since I hung feeders with black-oil sunflower seeds, and corn on “the twister.” As I type this, a squirrel is trying to reach the ear of corn on the “twister.” Oh so funny to watch the activity!!)
Three Quarters & Counting 
Boy, that’s odd. Squirrels couldn’t move the switch, but surely the lights on all that time since you last used it would have been noticed sooner. I hope the battery will take a charge. Some of the auto parts stores and places that sell batteries, like Walmart, will put them on a heavy duty charger for you and test them for free.