Agreed (on the leak potentially being nearby). I would drain the coolant & screw in an NPT "tank valve" (less than $3 at O'reillys) into the cooling temp sensor port at the top rear of the block & with shop air pump it up to 15 lbs & see if it holds. You can spray a soapy water solution around potential areas to see if it starts to bubble (for external leaks) & there is a "bubble up" (iirc) solution that reportedly bubbles more readily but the dish soap in some water has worked for me. EDIT on second thought I would pump it up to just 10 for a start.