Somehow the (upper) oil plug worked itself loose while I was driving and pretty much emptied all engine oil. I never saw any red light on the "dashboard" and I don't even know how long I drove without oil.

I happened to notice an odd sound while driving and pulled over, shifted to neutral and heard a quite unhealthy sound from the engine when I started to let go of the clutch. I killed the engine and soon found the problem. Long story short: I fitted a standard stainless M8x25 screw with washer and topped up the oil, using about 2 litres or so. Engine runs nicely now and I can't hear any odd sound or feel anything different after this incident.

Question is: how the F could this happen? I mean I know I must've failed to tighten the oil plug sufficiently after last change (right before winter storage, this was first/second drive after storage), but shouldn't some red warning light have alerted me to the problem BEFORE it got to the point that the enginge was sounding weird?

Also, is there something that can be done as far as checking for potential damage or should I just hope for the best? I also assume that Yamaha's oil plugs aren't enchanted or anything and that the stainless one I put there instead doesn't really need to be changed.

i just checked on my tenere and there's no red oil pressure light coming on.
just some sort of a high temp warning light... hopefully you were just in time before the engine actually ran without oil.
if it runs fine now, i'd run it for a few km so the engine gets nicely up to temp and drain the oil and filter to see of there's any metal flakes in the oil.
if you just put fresh oil in, you can catch it in a clean oil pan and re-use it if all looks good.
fingers crossed.