The superintendent found the bomb on return from a several-week vacation Jan. 22, Italy's ANSA news agency reported.
He opened the package, but the battery failed to trigger an explosion, Osapp Police Union Vice Secretary Mimmo Nicotra said Monday.
"Fortunately the parcel did not explode due to the length of time between when it was delivered and opened," Nicotra said.
"This is a serious incident that takes Sicily back decades. The State must immediately implement prison-staff increases and improve facilities," Nicotra said. "For now, we can thank our lucky stars, but how long will that luck last?"