ignition switch question

[tom61]

The ignition switch on my boat is two parts: the keyed ignition and a button for the starter. I inadvertently left the key on a couple of days ago after the engine was stopped and discovered that the battery drained overnight. Which I admit surprised me a little. There were no electric loads turned on anywhere that I can see, except for the bilge pump which runs off the same battery, but it's independent and didn't have a lot to do, either.

Without the engine running, I can't think of a route to complete the circuit, except maybe for the ammeter. So my theory is that the battery discharged through the ammeter and the coils of the generator (old engine, not an alternator).

My questions: Am I ignoring another obvious route to ground on a stopped engine? And: Is this a sign of a wiring problem I should look into? Or is this just the normal punishment for this kind of carelessness? Opinions welcome, thanks.