So two days ago I went to a party in the woods and parked the bike on the side of a narrow gravel road. I was concerned that any passing drivers might not see me in time so I placed the ignition key in P* position where you have parking lights on. I left the light on for about half an hour before we moved away. Bike started fine and I drove home.

Yesterday I tried starting the bike but it would not turn. No sound from starter or solenoid click, nothing. Neutral light was glowing but as I pressed ignition it would slowly fade. Battery was showing 12.65 volts.

So today I disengaged the negative pole on battery and tried to get a reading on a multi meter. There has to be a short circuit somewhere.

I connected negative battery with some part of the frame.

In OFF position I got 0.48 volts. Is that normal?
In ON position same voltage as the battery.

After i checked some connections under the seat I couldn't find any issues. So i reconnected everything and bike started fine.  

When running battery reads 14 volts. With lights on 13.85 or something similar.

There was some rain the day before the problem. My theory is that some corrosion happened on the battery contacts or somewhere on the wiring but it dried till the next day.

Any tips on where to look to investigate further?

Now it works. But I will try later again.

Battery ground by the oil sight? Don't remember a wire being there but I'll have a look.

  You should have started by checking the battery voltage across the posts with the key off , that would be slightly over 12 volts if the battery is good. Make sure the screws on the posts are tight ,and the ground bolt at the motor case is clean and tight, this is found about 2" above the oil sight glass and about 2" in on the center case (heavy black wire like the ground post)    The bike running and showing 13-14.5 volts( again across the battery posts),  tells that your alternator is alive and well.  The fact that the bike didn't start could be that some water found it's way into the KILL switch ,if you use that switch to shut off the bike , rather than shifting into neutral and turning off the key, the contacts will stay clean.
  The .48Volts you saw I believe is normal , what you saw,  is a small draw from the decomp control relay which is always present even with the bike off.

''How'd you turn the light off with the bike running? ''

Either with wire cutters, or a hammer.

I live in Europe. Don't know about regulatory differences but light switch has off position.

Headlights working all the time for motorcycles didn't become mandatory until relatively recently.

I think that might have been it. Because when I removed the seat I cleaned all the contacts and connectors.

Yesterday there was another storm with heavy rain and hail at one point. Bike was caught outside. It was soaked later but started without an issue.
Need to learn to park it under a tree.

Thank you everyone for your suggestions!