lol, sniff test is a laymans term for the combustion gas test - garages have a container full of a dye that changes colour in the presence of exhaust gases. If it does this when gas from the coolant tank is used, its a sure sign the headgasket has gone.
The oil cooler is on the front of the engine where the oil filter screws into. If you think its this, you can disconnect both the coolant hoses and connect them together to bypass the oil cooler. If no coolant is lost, jobs a goodun'
Sounds like you are losing coolant from the coolant flange on the side of the head. Have a look at the complicated looking one on the front of the engine too. Both are common for the seals going and leaking water when hot.