Hello, Long time lurker first time poster.

About a month ago, I finished a top end rebuild of my 09 MCS, I got the head done from a reputable engine builder and did the rest of the work I did myself and this was nothing new to my although it was my first time digging into a mini.
List of things I did (coolant related)
head gasket
oil filter housing/cooler gaskets
thermostat gasket

now that's its been rebuilt it runs great! after bleeding and filling according to procedure, I drove the car 300 miles. I was washing the car and decided to check the oil and coolant. oil was perfectly fine, but the expansion tank was bone dry (no overheating)?? so I filled the expansion tank, over the maximum because I figured it would overflow and drop to the desired hot level. but now it randomly loses coolant. if its hot or running the coolant level seems ok, but if it gets cold it gets low, sometimes not visible in the expansion tank but barely below. id say I've gone through 3/4 gallon of coolant and I kind of feel like all of the coolant is being overflown. I should also mention I live in Vermont where the ambient temps are quite cold, I don't think this would make the correct coolant level that low though.

things to consider.
-I pressure tested the system, it dropped to approx 19 pounds after being pumped to twenty, I let it sit for about three mins. I don't think there's any leaks
- it almost seems like it the level likes to sit at the bottom of the expansion tank which is bazaar to me.
-I once opened it hot and it seemed as if the cap was holding pressure but im not sure if it was enough to have the system properly hold coolant.

im kind of puzzled at this point, is it fine and I should just let it go? I don't know and if any one has any advice I could really use some suggestions

thank you,
-Ben

 

Check around the transmission in case the coolant is collecting there. Also, there was another member with something similar and it ended up being his transmission cooler (for automatic transmission). also check around the water pump and pipe, it could be dripping and drying faster than reaching the floor.