Land Rover Coolant Leak LR3

it's important to conduct your own testing if a mechanic tells you you need a new head gasket. I was told I did and I conducted my own due diligence, tested for combustion gas and found none in the coolant...checked for leaks into the oil and nothing...those are signs of a bad head gasket...still could not find the leak after the mechanic told me it was internal and needs a new gasket...so I bought a pressure tester and found a hairline crack where it was slowly seeping thru in the expansion tank!