Reminds me of an incident driving down a busy road towards some traffic lights on a busy, sunny day - lots of traffic & pedestrians around.
As I drove down towards a busy junction, out of the corner of my eye I caught a glimpse of a Weimeraner dog, bright-eyed, floppy-eared and tongue lolling, apparently loose next to a busy junction.
Instantly, I slowed down just in case it ran out into the road, but on turning my head, it proved to be a large patch of grey moisture on the pavement, which somehow in the dappled sunlight my brain had "filled in" the necessary detail to make it look like a dog.
For an instant it was like one of those "rabbit/duck" optical illusions, and enough to make me prepare to stop. I was quite amused by the illusion and the "added detail" that was filled in, and hasten to add that it's not something that happened since - maybe just a combination of dappled sunlight and a heightened sense of danger due to the amount of traffic and pedestrians around that made me "look" for a dog just in case.
I might also add that I have two cats, with the accompanying bizarre expeditions into cat psychology - the chasing of "nothings" and fixations on tiny moving/non-moving light patterns and pieces of fluff.
I'm amazed at their patience with small visual things that disturb them, presumably as an evolutionary result of waiting for extended periods outside mousehouses, waiting for things to emerge.
There's a small patch of paint next to my bed a few feet off the ground, that's been absent-mindedly "touched up" with a single brushstroke of eggshell paint on a background of matte emulsion - in the semi-darkness of my reading light I'm quite frequently disturbed by one of the less sane of our cats repeatedly leaping at this tiny patch of paint, presumably because it differs sufficiently in brightness and contrast to make it worthy of investigation.
I'll quite frequently get a "start" whilst trying to have a quiet read, by the cat unexpectedly leaping at this seemingly inconspicuous patch of paint not far from my head, with an accompanying alarmingly-elephantine "thump" as she returns to earth. You would think that this would provide only a moment's amusement, but no, she has to be firmly told off for her to leave it alone, and will decide to have another go a couple of days later. Something about it, whilst seeming completely insignificant to me, and after a considerable number of unsuccessful investigations on her part, seems to make it a cat magnet.