Dear Baldur,
Standing in the middle of the hall or a nearby room crying loudly because you don't see me is Not Cute. Especially when you just left me and I haven't moved. I know you have object permanence; you never fail to find the food dish. So please remember, when you walk out of the office while I'm at the computer or especially when you jump off the bed at 3AM, that you know how to find me again without asking. Can you take care of that for me? Thanks.
He's been doing this for a year or so, so this is not just the effect of losing Erik and Embla. Google tells me that this behavior can mean:
Hyperthyroid. Yes, but it's being treated and his last blood test was at the end of October. So that's not it.
Going blind and/or deaf. I don't really know how to tell, given that I can't tell him to just read the eye chart for me. No visible clouding in his eyes and his pupils respond to light changes. He followed a laser pointer for me a couple days ago, but maybe light is different. He doesn't miss when jumping up on things or people and I haven't seen him stumbling or walking into walls. And he seems to notice activity in the kitchen. So I don't think it's this.
Depression. Don't know how to tell or what to do about it, though.
Dementia. Yes, apparently cats can get something like Alzheimer's. Whee. Rumor (ok, one web page) says there exists medicine for this; I've asked my vet for advice and she will get back to me.