Not insane, or a plane, or tears. Or storms. Soundless flash.
It was for sure some sort of electrical phenomena that happened with your retina. Most likely jostling from the bike ride, causing the vitreous humour to bump or shuffle the retina slightly to where it stimuli the motion as light, causing a perceived flash.
Could be, but it happened on the same day my grandmother died. Coincidence?
Yes.
I'm certain you only noticed it on those days because you were left to muse to your own thoughts, and thus weren't thinking about much else, causing it to be much more noticeable.
They can happen quite frequently, but are usually the most noticeable when it's dark outside, for obvious reasons, and also when not distracted.
Therefore: It wasn't a deity caused event. God is too busy sticking tentacles in our necks for reproduction to answer calls for visions.
