Finding mistakes before they bite you
The importance of searching prisoners before turning them over to another officer/jail cannot be understated. In fact, many street officers conduct only a visual search or a frisk search on their prisoners at the point of arrest.
Even prisoners in custody often get only a cursory search. The bottom line is that you need to conduct your own THOROUGH custodial search whenever a prisoner it turned over to you from another agency, left in a non-sterile area, or who is being turned over to another jurisdiction.
Unfortunately, trust is a wonderful thing but has no place in law enforcement. You want to find mistakes before they bite you physically in terms of being assaulted by the weapon that someone missed, legally in terms of missing contraband found later that you end up being disciplined for, or psychologically in the case of knowing your poor searching led to another officer injury or death.