At least 14 years ago, we got Jess, our tabby cat, and while later we got Lucy, a collie cross spaniel. They were never best buds but Lucy would whine at the door when she heard cats fighting-scared it was Jess. They used to play together too- Jess would paw Lucy's nose. But unfortunatley, last week Lucy died after all that time living with us and Jess. She has always been more of an "outdoor-sy" cat, but since Lucy's death, Jess has been staying outside a lot more than she used to,. And when she's inside she will act distracted and I haven't seen her eat since either- and she's usually a greedy-guts!

Is it that because Jess was taken away from her mum too early that she always saw Lucy as her mum and is missing her even though they didn't seem close??

If one of my cats stays out overnight first thing in the morning my other cats are out on patrol looking for them. I think the cat will be missing the dog, perhaps not in the same way that we miss animals, but still missing the dog nevertheless. The dog was there for as long as she can remember and now the dog isn't. She may be outside more as she is looking for the dog?
One of my cats was very subdued, not eating, occasionally picking at his food, not grooming himself after his litter brother died, it went on for so long I took him to the vets! I ended up getting him a kitten (the vet said it might help him, it was a last resort as he'd lost so much weight and looked a total mess). The day the kitten arrived he was totally changed (he made a very good mummy, cleaning the kitten, carrying her around, bringing her to visitors to show her off - think he's a bit confused, lol) and started to act more like his old self. The vet did say that he'd never seen such extreeme behaviour last for so long in a cat, they do usually look for the missing 'family' member but stop doing this after a while.