It's clear that someone close to the characters died, but they never seem to mention that person.
In a preemptive Put on a bus, a character was killed off before the show started. Still, you'd think the surviving characters would bring up memories now and then. Examples - Toy Story. What happened to Dad? Forgivable since the humans are side characters. - Averted in Arrested Development, where the family regularly mentions Michael's dead wife.
Hello, Unknown Troper. You'll need to get known to lend a hand here.