Today (as always) opened engine compartment to look inside after a day of sailing (and then driving
when the wind died). What do I see, but a nut lying down there below the engine. Well, I don't normally have nuts there so I knew it fell from something. Not a good feeling after running for an hour at high rpm. Long story short, after inspecting it I realized it must be from the alternator somewhere, and sure enough - it was the nut that held the lower alternator bolt. It's a good thing this bolt is pretty long and alternator tension holds it - but still, not good at all. I did slacken it a few weeks ago when I adjusted belt tension, but then I also tightened it. Re-tightened again as hard as I could and will watch it now.
So, check your engine - you never know what shakes out.