coolant leak issue - couple questions

[MinneapolisJeff]

working through a minor coolant leak in my 2003 Z4 3.0L, just over 75k miles

oddities I've noticed:
-with the temps warmer, and with regular driving, not seeing the leak at the bottom of the engine bay each day i'm checking
-the car sat for about 2 weeks and had a small puddle on the engine underneath cover, in the middle, but no leak since then

so my questions are this:
1) if it were the water pump seal failing, wouldn't I see coolant leaking as I check daily? not that I don't expect to replace it soon, trying to pinpoint my work at the moment to a more exact area
2) brand recommendations for thermostat & hoses? the usual sites I go to (FCPEuro, RMEuropean, PelicanParts, etc.) have prices varying quite a bit, depending on brands (Genuine BMW vs. other suppliers)

when I got the car out of cold winter storage, it did throw a P0128 code (both this year and last year) but I've cleared it both times and it hasn't come back in either instance, which is why i'm focusing attention on the thermostat

[XMetal]

I had a similar issue on my X5 with the same M54 engine. Not sure how the expansion tank is mounted on the Z4 with the M54, but my problem on the X5 was a failing Oring on the expansion tank mount. The tech pressurized the system for a long time and didn't see a leak. Then he left it pressurized over his lunch break and came back to a small leak at the expansion tank mount. It's been good for a couple of months now after replacing the expansion tank and mount - use only genuine BMW tank and mount!

[Clearancediver]

When I had a leak on the engine to expansion tank hose it was coming from where it plugged into the expansion tank… I pulled/removed the hose from the expansion tank… cleaned both mating/engagement port/hose surface/o-ring etc… then did a light lube of silicone grease on the surfaces and o-ring and plugged them back together. Never had an issue going forward. I did a cooling system refresh a couple years later and did the same for the new hoses/expansion tank.

When I say light… I mean just a film to assist mating. Not going to affect the coolant at all. Never had an issue in the years following and I’ve had the car now for 11 years… My car neither leaks oil or coolant.. nor has any oil residue in the engine bay… I did the oil filter housing o-ring and the valve gaskets/crank case valve replacement bc etc because…. I hate oil leaks/dirty engine.

[Piper1]

When my expansion tank went there were no warning signs at all. I had done tech sheet the night before an otd and all was good. Pulled into the paddock and after coming from check in found a nice stream of coolant. They can have pin hole leaks that are hard to detect but this was mine when I pulled it. If it’s not been done with the age and miles it’s a good idea to do it even if that’s not the culprit.

[MinneapolisJeff]

Thanks for everyone's feedback, still haven't seen any leaking return, so while I go out of town over the next weekend, I'm tabling my decision on which parts to replace for another week. Such an odd issue that seems predicated entirely on the contraction/expansion of parts while stored in winter.