The battery was not holding a charge

The battery was not holding a charge. I replaced the battery. After a few days of using the truck the battery would be dead. I replaced the alternator as well. The battery will still run down. I had the alternator and battery checked. They were both good. The battery will still run down after a few days. Do you think that a short is in the system. And how would I check for the short. I had a mechanic tell me that the positive battery cable may cause this problem. Is this possible