don t the D-series motors have an iron head? if so, likelihood of a warped head to accompany the maybe blown HG is low
if you don t see any physical leaks anywhere on the motor, i m gonna guess your coolant is going the way of the combustion chamber, especially if you don t see any white smoke from the tailpipe as you say
but of course, I wholeheartedly agree that getting a new rad cap should be your immediate course of action...they're cheap but they do fail and leak overheated/boiled coolant that can't stay pressurized