If he was missing for a few days, if he was outside or had access to your garage, if you have one, I would think that the chances are greater for him to have ingested antifreeze. Cats and dogs are drawn to antifreeze, because it has a sweet smell/taste to them. Sometimes, the symptoms can be delaying in appearing with this sort of toxic ingestion.
Vestibular Disease is a malfunction in the vestibular apparatus, which controls balance/equilibrium. Walking drunkenly or stumbling is caused by a nerve that runs from the inner ear, to the brain. I clicked on the link I provided and it worked fine, but I'll re-send it, and hopefully you'll be able to view the article.http://www.petplace.com/cats/vestibular-disease-in-cats/page1.aspx
If it's still a problem, please go to www.petplace.com
and put 'vestibular disease' into the search box.
Cats who have ear infections usually do exhibit pain and/or discomfort in the ear(s) by pawing at it or rubbing it/head shaking, etc. However, it's still possible for him to have an inner or middle ear problem and not show any symptoms of discomfort. The thing is, that the 'balance center' of the body is located in the ear and if something is wrong, this is usually where it will originate from.
Usually, vestibular disease will resolve on it's own within a few weeks, but if the dizziness causes nausea, vomiting, inappetance, the vet will prescribe an anti-nausea medication to help counteract this.