We're constantly chasing leaks (air, oil, hydro and fuel) on our equipment out here. They all get tracked down to damaged hoses, bad packing (seals or o-rings) or damaged seating faces. You may have a warp, crack or scratch in the filter mount, but more likely, it's just a crack/pinhole in a line.

Tank to IP, it is a simple low pressure system. Have you swapped all the soft bits yet? Oh, and I totally understand how frustrating it is not being able to nail down the cause... mind starts coming up with all sorts of inventive ideas...