Officer BUBBZ
These radio I D's you R seeing are the "Private Call" feature of the MARCS system.
One radio, talking directly to another radio, useing thier particular I D numbers assigned to the radios themselves.
When you're hearing both ends of the conversation, they're probably within close proximity to each other, useing the same repeater tower.
When you're only hearing one end of the conversation, they are some distance away from each other, useing two or more repeater towers, bounceing around. Example: Somebody in Northern Ohio, talking to another, say southern Ohio.