Coolant Loss

[openwater]

New revelation.... After removing the reservoir cover (looking for the bleeder valve) I found tons of antifreeze residue and a small overflow port which runs down through the center of the tank, also caked with moist residue. I suppose it's there to handle and oil or coolant spills while filling...

Is it possible that my pressure cap is shot? After pulling the carpet and closely inspecting the tank it looks in very good shape, no residue on the sensor or around the base. Also, there remains about 3 inches of coolant in the tank, below the view of the check window, but no evidence of further leakage since it cooled down 3 hours ago.

I know the easiest way to check would be to get a new cap, fill the reservoir and go throughout the bleed process... does this make sense though?

Would my topping off last Saturday make this failure more likely?