One of our cats was "put to sleep" a couple days ago, not yet 48 hours ago at this writing. She was just short of 15 years old and was in declining health.
That leaves us with one other cat. They were together a lot. The other day I went into one of the bedrooms and they were both on the bed. The cat we lost was stretched out sleeping. And the other cat had his head up on her haunch, like a pillow, also sleeping. I would've taken a picture of it, but by the time I got the camera they had shifted. It was sweet looking.
Since they were together a lot -- neither one had ever known life without the other -- I'm thinking he's in grief, wondering in whatever terms a cat wonders about things where she is. At least I'm attributing that to him. He's acting a little funkier than usual, so it makes sense.
How could any creature spend 15 years with another creature and no wonder what's going on when there's a sudden change?
But of course there's no way to explain it to him. It's just a matter for him to work out on his own. Except we're very tender with him and think it helps to give him some tender care.