It will depend alot on how they were killed also. The place where I get my rodents preps/kills anything larget then hoppers by breaking the neck. This is very quick and effective but does often lead to some leaking of blood from the nose or mouth on the freshly killed rodent.
Furthermore if they are frozen right after this it will not necesarily leak before freezing but may start once defrosting starts. Because of how they kill them where I get mine I'd say at least 50% of the rodents I get have visible blood on the snout or leakage(drop or two) on whatever container they are in.