Vet : Hi there, vet Andrew here.
Vet : I'm sorry to hear about what's happened - you must be worried. If you haven't done so already, think of all the places around your house and the local area you've seen your cat, and search each one systematically. While you search, run through your mind any potential access he might have had to something poisonous - it could be slug pellets, antifreeze, human drugs such as paracetamol or even any treatments you've recently given him.
Vet : Cats often hide away when they're ill, and some come back once recovered as if nothing had happened, so his failure to return may not necessarily be such a bad sign.
Vet : As I've indicated above, my first suspicion is a poisoning, so once he comes back or you find him, take him straight to your vet with any information you have on poisons he may have had access to.
Vet : You haven't indicated his age or whether he has any known health problems, but if he's very old then he could be showing the first signs of something like kidney disease. In any case, your vet will be able to give a full assessment.
Vet : I can't give any more advice based on what you've told me, but being sick can be a response to so many disorders, from hairballs and worms to more serious problems. Dilated pupils can be simply a sign of stress, discomfort or fear. The key here is not to panic. I wish you all the best. Vet Andrew.