So.... what the actual fuck.
While my bike was parked outside a friend's place in the inner city (backyard/parking lot of the apartment building, to be precise, not closed off but also not visible from the street), someone removed the axle from the front wheel hub:
Furthermore, the rear wheel axle was screwed loose as well, but not taken. The bike had/has quick releases on both wheels.
The bike was secured with two locks (one through the front wheel and frame and around a massive pole, the other through rear wheel and frame). So I cannot say whether this was an attempt to steal the front wheel for the hub dynamo, or simply sabotage for whatever reason. In the latter case it may have been done by one of the neighours from the house for whatever reason, but that is only speculation.
I noticed the loose front wheel when I was lifting and spinning it to test the lights before setting off for the ride home, so at least I didn't ride it with the inevitable outcome of an accident at the first bump in the road.
Two other bikes right next to mine were not damaged. One belongs to another friend who was with us, the other presumably belongs to a neighbour and hadn't been there when I arrived.
He might have done it out of spite because I took "his" spot to park my bike, but as I said above, that is pure speculation.
Now, the fun of getting a replacement part...
Also, yes I have notified the police. There is of course only a slim chance of the culprit being caught, but this is potentially not only theft and property damage, but potential assault by sabotage of a vehicle (or whatever the correct legal definition might be). And a crime not reported is a crime not existing.
Besides, I hope to get the cost for the replacement part back from my theft insurance, and they of course require this.