Battery draining overnight

Now that I’m starting to work on my car again, I’ve been trying to figure out why my battery will drain to nothing overnight if it’s left connected. I’ve noticed when the battery is disconnected, the ammeter rests perfectly in the middle of the gauge, but as soon as I connect the battery it will drop to here (see picture). This is with the key to the off position and everything turned off. Doesn’t this mean that something is drawing power from somewhere? I’ve been probing around with a test light trying to figure out what is doing it but can’t find anything. I’m thinking about just wiring in a battery kill switch somewhere.
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