Are you sure that ocelots act like hostile mobs? I thought they were like passives and spawned with the world and didn't de-spawn?
The only mobs that do not despawn are animals (pigs, cows and mooshrooms, sheep, and chicken), and tamed animals (tamed wolves and ocelots). Untamed wolves and ocelots despawn.
If your cage had any stone, cobblestone, or stone bricks in it, it is likely the silverfish went into it. If not, it despawned.
The only mobs that do not despawn are animals (pigs, cows and mooshrooms, sheep, and chicken), and tamed animals (tamed wolves and ocelots). Untamed wolves and ocelots despawn.
If your cage had any stone, cobblestone, or stone bricks in it, it is likely the silverfish went into it. If not, it despawned.