Water leak?

[wildcard]

I have a 300 I-6 in my truck. For a good while now I have had to add a cup or so of coolant to my radiator about every 2 weeks. I changed the water pump and radiator this past summer. Not just throwing parts at it, they needed changing.
Also the heater core and hoses were changed a year ago.
I've taken a flashlight and checked everywhere for any sign of a leak without finding any.
I've been noticing lately that it looks like a water/oil mix on my oil filler cap when I pulled it off. I check my oil, water and brake fluid every week even if it doesn't move. There is never a sign of water in the oil.
Is it possible for water to access inside the valve cover without showing up on the dipstick? Or do I have a coolant leak that I can't find and condensation in the valve cover?
Thanks for any help.

[jzjames]

I'd look for some drips at the hose connections.

[Ranchero50]

Dumb question, but if you don't add coolant, does it eventually stabilize or just keep going down? Non overflow radiators will puke some out when hot but will stabilize above the tank bottom.

Mud in the cap is from condensation, either naturally from weather fronts of a coolant leak internally.

[wildcard]

That's not a dumb question. I should of mentioned that it keeps getting lower.
If the mud look is from condensation, then I can live with that.
I've looked at all the hose connections and there are no visible drips. It's always parked in the same place and never been any sign of a leak on the concrete. Even if I take it to a store when I come out, I look for signs of any type leak when I walk up to it. Just an old habit.
And the floor is never wet so I'm sure it's not the heater core.

[bwlyon]

With engine running sniff the exhaust. Does it have a sweet antifreeze smell? If so you have a crack somewhere. Also, check the freeze plugs, with the age of these trucks the plugs made out of steel can begin to weep if they are corroded enough.