:-( sorry to hear about the older kitty...cats will hide when they're ill..my Dido disappeared for three days, very unlike her. I spent the entire three days hunting for her and found her hiding under a bush during a downpour.
It turned out that she was ill and was hiding. Shortly after I brought her back in the house, she attempted to stand up, then collapsed. I hydrated her through the night and got her into the vet first thing in the morning. Turned out she has a gastrointestinal infection and that if I had waited any longer she would have died.
A few days and nine hundred bucks later, on IV antibiotics, and she was ready to come home...but I still had to pill her for two more weeks after that. Oh, what fun. She was feeling well enough to turn both of my arms into road maps, and she terrorized my dad for a few years after that. :-)
...funny as hell... I'd be downstairs studying or something or other, and I'd hear a MEOWWWW!!!! ... a stumble... then my dad yelling CAAAATTTT!!!!
Other funny thing was, is that until Dido, Dad HATED cats ... but he loved her.... and when she disappeared, he was looking for her, too. Dad could be a cantankerous old coot, but he met his match with Dido, and when I wasn't around, she'd be taking a nap with him.
Anyhooo, give your kitties a pat for me :-) take care