These caps don’t just fall off, or hanging on by just hope as mentioned above. This is user induced error by not properly inserting the dipstick and then incorrectly fitting the cap. If it stayed on by hope this tread would be a lot longer and older…
This bike has been out for 5 years now and this to my knowledge is one of the first cases.
When did you last check and or replaced your oil?
I've lost two- yes, operator error most likely, but hard to say. I'm very careful now to make sure that it's oriented correctly and that all three tabs are properly engaged.
I have an idea when it might have flung off. Crossing some RR tracks were angled, so I transitioned on the pegs to cross more straight on. I caught a little air going over and the bike twisted a bit when it landed. I remember feeling my left thigh pushed over. It might have twisted the cap and jettisoned it on landing. I'm going to go look around there today to see if it is there. But no, I didn't leave the cap loose.
It's a challenging RR crossing on a good day. When one of the ,Type A motorists as I call them, rushed behind me I throttled up. I've been rear ended before and watch my mirrors.