A friend has 2 male cats she got as kittens 3 years ago. They've never been separated, were indoor only but she lost that battle last year. A few weeks ago one didn't come home and was missing for 4 days, 3 nites. During that time, the one that was home was very lethargic, wouldn't eat, drink, or go out. When the missing one came home, he went to his brother and started washing him and he was back to normal within half an hour. Her question is, is it possible that her cat knew his brother was missing and went into mourning?