If it's getting the benze from a leaking head gasket, it will usually show up because there will be bubbles in the coolant and can be seen through the radiator cap. Sometimes a coolant pressure test will not show it if it's a small leak. It also may seal itself after a few heat cycles. A re torque may be a thought. Also, one of my first jobs was re coring radiators, coolant system work. We used to use powdered aluminum stop leak in everything that we worked on, even with all new parts, been told that car manufacturers do it too. Just a thought. I've had them leak and tore them down to find the leak was due to the surface marks from having the head, block resurfaced when done on a mill. That was when using steel shim head gaskets, never seen that with fel pro blue, or similar.