This is in reference to Marine Corps Detachments stationed at Embassies overseas, which maintain Detachment Property Logs of any and all property belonging to the detachment.
The alternate usage of Det-Property refers to any female (in rare cases, male) who has had "relations" with three (3) or more of the Marines assigned to a specific embassy detachment; this is regardless of rotation and carries over to new Marines assigned to the detachment.
Once a person has achieved Det-Property status, it is non-revokable and in perpetuity.
"Thank you. Who's that hottie over there?"
"That's so and so, she's det-property." "She's already banged three Marines here."