We got our two cats as kittens, brother and sister, 14 years ago.
Last summer my girl cat died after a short illness, but since then her brother seems to be calling for her in the house. It's usually just after he comes in from outside, he prowls round meowing really loudly, and it's a really strange call, almost human. It really does sound as if he's missing her and is cross with her for staying hidden for so long!
I wondered if anyone else had noticed this behaviour after losing one of a pair.