Cats might not understand death the way humans do, but they sure understand when someone they love is not where they should be.
Maru graced us with his presence for 18 years, and then left us on September 6. When we announced his passing, Mary Ann 02 asked about his little sister Hana. Hana had lived with Maru for all of her 12 years, and felt his absence deeply. She and the youngest cat Miri are clinging to Maru's favorite objects and inhabiting the beds where he napped. So many people inquired about the younger cats that mugumogu posted this video update, with the story told in captions.
A week after Maru died, mugumogu posted about Miri's 5th birthday party (in which a plate was prepared for Maru) and hinted that Miri may have seen a vision of Maru in attendance. A further update tells us that Hana is still looking for him.
I doubt she ever understood what had happened.