Call me crazy, but it looks like resources that an enemy builds or is in the process of building, which suddenly end up in my zone of control via either a well-placed outpost, settlement, or a successful city assault, remain the property of their original owners. Until I move a unit of my own over it. I suppose that this is intended behavior, but it isn't clear visually that I'm expected to move units over the resources to claim them, because there is no icon like there is for some of the other interactions (speaking to other units, attacking enemies) when I mouse over the resources while moving units. Nor is there any text in the resource info window which directly suggests that I can take it over.