Kerry, I was experiencing loss of coolant a couple of years ago also. I’m assuming you are not finding coolant under the engine or in the bilge that would indicate a leak.

When I checked my heat exchanger I removed it and pressure tested at home. I sealed one end of the heat exchanger and pressurize by the other end using my shop compressor to 2 atms. I then submerge it in water and checked for leaks.

Kerry-
We had a slow coolant leak that drove me crazy. Every few hours, I had to replenish about a cup of coolant.
 Ultimately, our slow leak was located at the hose nipple on the exhaust manifold reservoir.   The nipple had a small burr. (The nipple is an integral part of the manifold casting and the mold seam and wasn't totally smoothed  when it was manufactured). I had to sand paper/grind it with a dremmel to remove the flashing. This burr  prevented the hose from making a good seal regardless of how tight the hose clamp was. A search of the C36IA archives indicated that others had seen the same manifold burr issue.
You can lose antifreeze in many places, but I would check that spot to confirm. Dab a paper towel against the hose connections to identify leaks. For slow leaks, the coolant can evaporate before it leaves any trace and will only be apparent in the immediate hose connection area. 

Good luck

Kerry, I had that same problem for a long time. I would find 2 table spoon of coolant in the bilge at the bottom of the engine. I replaced all the hoses but the problem persisted. It turned out to be the fill cap leaking. Since the engine runs hot the leaked coolant evaporates so there is no trace of it. If you have not replaced the fresh water hoses - I recommend do that now. There is a kit you can order that includes everything you need including hose clamps and filler cap.

If you've eliminated external leaking and made sure it's not making it into your oil, then checking your heat exchanger for a leak between your fresh water and salt water circuits is called for.  Can be done at home, but kind of a science experiment involving a shop vac, adapters, plugs, and soapy water.  You might phone around and see if a radiator shop can handle the job for you.

Mine did that.  Coolant was going out with the raw water in a slow leak. One of the tubes separated from the tubesheet down by the anode. The outside of the heat exchanger was green with corrosion, so it wasn't much of a surprise.  It was part of the previous owner's deferred maintenance plan.