random unequal pupil size in 6m old cat

Hi so we have 3 cats, Jet(male 9 years old), Holly(female 4 years old) and Zeke(male 6 months old). 

All 3 are healthy as of their last vet exams, all 3 up to date on shots, all 3 acting normal(eating, playing, sleeping, cuddling, general cat stuff), but Zeke has 1 pupil that is much larger than the other.

We first noticed the problem about 1 week ago and promptly took him to the vet, blood tests, physical exam and an in-depth eye exam all turned up clear. The vet said since he is young and his blood test was good that she can safely rule out diseases that would cause the condition (anisocoria), and since he is acting 100% fine that it doesn't seem to be anything causing him any pain (infection/tumors/trauma/toxicicity)... So she gave us some medicated eye drops just in case he has a scratch or something on his eye that she couldn't see.

Anyways, we've been giving the drops as directed and he has been fine, no dilation or anything since the visit, no mood or habit changes, nothing... Until today. This morning he woke me up as he usually does, meowing in my face for breakfast, so I got up and fed them (about 8am) and his eyes were fine (have been checking them regularly). I ate my own breakfast, then went and ran some errands, came home at about 11am and didn't notice him any different, put away groceries, ate my lunch and went about my business. After about 30 minutes or so I noticed that one of his pupils is HUGE in the semi-dark room whereas the other is dilated to the proper size. He is still acting normal as are the other cats so no other symptoms than the large pupil...

My question is if anyone has experienced this issue before with no diagnosed problems other than the anisocoria indicating that he may be getting into something around our house that is causing it to happen temporarily?

This is always a scary first symptom for me, as it connects neurologically, and can be from FIP, which of course is always feared due to it's inability to cure.

I would seriously be considering further testing, an MRI perhaps, this really does need an explanation, and surely you are worried.

I called the vet, she doesn't seem too concerned since there are no other symptoms at all and seems to think he may be getting into something around the house that could be causing his blood pressure to spike temporarily? Because now after about 2 hours his eyes are almost back to normal so I'm inclined to agree with her. She again said if it continues or he develops more symptoms to bring him in for more thorough testing.

So in the mean time I'm continuing to monitor him and have moved some chemicals from a place he can get to to a higher and closed cabinet, maybe that's what it was?

Hard to know really, how about house plants giving him a temporary effect??

We don't keep any plants in the house, he might have even gotten at my caffeinated drink while I wasn't watching, he will literally take a taste of anything lol...

Meh, not thinking that would cause just one pupil to react.

I don't honestly know other than what I first posted, but know it would scare me, since it is directly connected neurologically.

Might want to think about another vet's opinion?

Not understanding why your vet seems so casual about it.

Can I ask you, have you had kitty tested for felv?

The more I think about it, there aren't plants or toxins that would only cause 1 pupil to dilate.

No, this really does need further investigating.

An x-ray first, a CT scan, an MRI, an ultrasound, these are the options that will test his issue. 

Possibly recent experience but having a cat with one eye dilated should be enough of a symptom to warrant further investigation.

We were told it was our quick action and observation of something going wrong with our cats eyes being the reason she still has partial vision. We have sat through two ophthalmologist exams and fully understand the damage done to our cat and that it is not reversible. If you cat is having a detached retina that can be fixed if caught in time. And the pupil itself could be a sign of a medical condition.  Damage to a cats eye no matter the source can cause permanent damage and blindness.