Replaced waterpump/thermostat in 2011 335, and now have an inconsistant loss of coolant... The leak appears to be coming from the general vicinity of the water pump/thermostat. As far as the inconsistancy goes, there are times when it does not leak much at all, maybe a few teaspoons a day, and at other times, it seems to lose pints a day. my typical commute is 45 mins to an hour, one way.
got that, but the loss varies on same drive, same temperature outside, everything similar. wonder if the co;nnectors might jiggle from looser to tighter. and so on. Another thought I have, is variance in presence of air in system, as I am having to add coolant every other day. Anyway, I do appreciate your input! Thank you!
Probably getting "analysis paralysis" here.

Now I tend to err on the side of over-analysis myself, so I'm NOT throwing stones. SOME analysis is needed to know what to focus your examination & testing on so you have a "plan."
However, in this instance, "vapor pressure" may be all you need to know (you can use that to pressurize the cooling system, rather than using a pump). Simply running the engine to get the coolant to normal operating temp will pressurize the system, and you will get some liquid coolant dropping on the cardboard on the floor/driveway (presuming you have taken off the undertray/splash shield. Running the engine also has the plus of having the thermostat valve move through it's range of motion, and runs the coolant pump at different speeds
It would be MOST unusual for a loose connector/fitting to "tighten itself." Thermal expansion could have an effect, and parts NOT properly retained/secured could "flop around" varying the size of the leak opening. I have never dissected either the coolant pump or the thermostat, so have no idea if thermostat position or pump speed might affect leak rate. Also "heat soak" can cause the leak to occur, or be worse, AFTER shutdown following 20 to 30 minutes of operation.
What YOU and I both know is that you say you have a leak, and it appears to be in the right-front of the engine compartment near the pump, thermostat, and lower radiator outlet. So why not just try to find the leak?
My Plan:
1) Get the front wheels on ramps;
2) Remove the Undershield (look for coolant in the shield -- leak, or at least bottom of leak path, is above ;-);
3) Examine ALL hoses, fittings & components in that area carefully with bright light for any sign of coolant; note suspected leak point per that examination if applicable;
4) Take photos;
5) Clean/Dry all hoses, fittings -- don't forget the reservoir above;
6) Place cardboard, newspaper, etc. below pump area;
7) Fill Coolant Reservoir, inspect cap seal, tighten properly;
8) DISCONNECT RADIATOR FAN CONNECTOR but remember that you don't want to run engine for more than ~ 5 to 10 minutes;
9) Set "Hidden Menu" to 7.00, Engine Coolant Temp (to read it on instrument cluster) or read that parameter with scan tool/ laptop connected to OBDII port -- monitor coolant temp every two minutes during engine operation;
10) Start engine and let idle, checking cardboard for dropping liquid;
11) If no leak, or only a drop or two after ~ 10 minutes and engine coolant temp reaching ~ 190, turn off engine and see if any leak THEN appears;
12) Get under vehicle with bright light & trace leak to source -- gently manipulate hose/fitting in that area to see if leak increases (wear goggles at least for that phase, to prevent injury if something breaks open and sprays hot coolant).
13) OBVIOUSLY: let system cool and release pressure before removing any hose fitting.
That "PLAN" is NOT meant to be taken as expert authority, just my best guess to date on how to approach, so any other thoughts/ critique are welcome -- purpose being to give Robie some ideas, just in case he's NOT already ahead of ALL of us.
George