I know some MINIs leaked coolant from the thermostat housing.

Yes, it might be worth finding another shop that can perform a pressure test on the cooling system. See if you can find a shop familiar with BMW's / MINI's or European cars in general.

So there was a visible leak when the pump was faulty the first time, the was a water leak around the pump belt.
Since then there is a really tiny of coolant that disappear every 1000 miles or so.
As far as I know my mechanic didn't put the cooling system under pressure using a specialized pressure tool, he just tried to find the leak by letting the engine running for an hour and trough visual inspection. After that he said first was the new pump probably came faulty from factory and then he said he was the radiator instead, which he replaced.
Honestly looks like he is trying stuff without being sure and at this point I don't trust him anymore, before dump the car in a landfill I would at least be more sure is a gasket leak.
are there more tests than can help? for example I can ask another mechanic to properly put the cooling system under pressure and check if there is a pressure leak?
As  i Said i did the chemical gasket leak myself and seems didnt find any issue, but I don't know if they can have false negative, the instruction didn't say anything about success rate.

Thanks!

Speedwago.. there  is a class action suits about this against  BMW MINI CVT tranny.   might do google search..   I will ask for you vin number.. to file a claim..  

Diagnosing a car issue remotely is difficult, diagnosing accurately is even more difficult.

If there are no indications of a physical leak like wet spots / puddles under the car, then it is 'possible' there may be a blown head gasket. If it is a small enough leak, it may be evaporating before it hits the ground.

A Blown head gasket can cause the coolant to:

leak into the combustion chamber and burn off (white smoke or coolant smell in the exhaust and sometimes creates bubble in the coolant)
leak into the oil supply (turns the oil milky)
leak outside the block

The only way to be 100% sure that it is the head gasket (that I know of) is to remove the head and inspect the head gasket.

Even the best auto mechanic will sometimes need to take an educated guess

Hello, I own a mini cooper 2004 model ( i purchased used, so I am not exactly sure about the history of the model).
I own the CVT model which is known to not be the best around, but obviously, I discovered it late.
Anyway I have these strange issues that don't seem to go away and I cannot pinpoint exactly what is it, maybe some of you can give advice. I like the car quite a lot and I don't want to get rid of her unless strictly necessary.
The problem is the following:
about one year ago, the car overheated while driving. The temp gauge went to red, I stopped the car immediately and towed it to the nearest garage. The car was with no coolant at all in the system. The strange fact the garage filled it back again and couldn't find any leak. Two days later I could notice a streak of liquid coming from the garage, the car was leaking again. This time the garage spotted the issue, it was a faulty water pump that apparently was about to fail so it was working at some capacity. So after the pump change, everything seem to be fine until about 6 months later I could notice the car was losing, VERY slowly, coolant again. I don't do too many km, I do less than 500 km a month. I noticed that every 2000 km it was leaking like a half a glass of coolant. So I went to the same garage which first suggested that it might be we installed a faulty pump ( that was still under warranty), but then while fitting the new pump the garage noticed that the leak actually came from a small plastic inlet of the radiator, so new radiator this time. Everything is fine until again 6 months later, when I realised it was missing liquid in the expansion thank again! Now the same garage says that most likely is a blown gasket but cannot say for sure. Also cannot say for sure how much would cost because they need to see the actual damage once the head is removed, ballpark they say is 2000-3000 euro.
Meanwhile, I have bled the system again and everything seems fine.
I have used one of those CO2 test tubes for gasket leak and let it on top of the expansion tanks and let the car running for about 15 minutes.... and the test didn't say there was co2. I am not sure how much you can trust those kinds of things off to eBay though.
I have also done other visual checks like milky oil cap, white smoke this kind of things and I didn't notice much.
I guess my questions are the following:
- is there a way to be 100% sure is a gasket issue?
- With a new radiator and a new water pump, what could be the cause of a leak?
- is the garage behaviour acceptable? they basically cannot say for sure, and cannot even do a quote and i feel like maybe there was no need of a new radiator after all?
- what do you suggest doing?

Thanks again for the help!