1998 Cadillac Deville Loosing water

Pull oil dip stick look normal? Or like chocolate malt?

Have system pressure checked and new cap if cap cannot hold 15 psi then coolant will boil and evaporate at normal engine temp

Any white vapor from tailpipe especially at start up in A.M?

At what rate are you adding fluid(qty/time). As stated in earlier post, have the system pressure tested, preferably cold, and block tested for combustion gases in the cooling system. Anything over 3 PPM hydrocarbons indicates a problem. I would take a guess you have a water pump housing(crossover) leak, or a leaking head gasket due to pulled head bolt threads in the block.

If your not seeing it on the ground(i.E. External leak), then I would say the engine is processing it. It could be at an amount across 1 or 2 cylinders or more, that you won't even notice steam at the tailpipe. How does it start after sitting overnight, does it run a little rough for a few seconds? If so, that is an indication that the difference in pressure is allowing some water into a cylinder. As you shut it down after warm, there is cooling system pressure still present. This will push the water past a leaking head gasket into the cylinder, since the compression pressure is not there to keep it back. And as the engine cools, and metals contract, the ability for this to happen increases. As it runs, there is still water being drawn in as well. A cold engine pressure test, and a block test from a cold start, if possible, should produce some results. If you have an external leak, it should be noticeable at the rate you are describing. You also mentioned overheat, this could make an internal leak even more possible.