Exactly the same trigger here. I had one just the other day at the pub. Sitting outside, under shade, but to my left was a bright spot. It’s weird how I can sense it arriving ... something about the quality of my vision subtly changes, and there it is.

Fortunately for me it isn’t accompanied by a headache. It’s just really unsettling. At least now I’ve learned to recognise them and I just try to chill out while it does its thing.

(FWIW, also a tremendous consumer of caffeine here. But this was at 17:00, a good 5 hours after my last cup.)