Engine Coolant level keeps dropping
First noticed 3 weeks ago the engine temperature fluctuating and the fan getting faster on my 2017 Q50 3.0T, checked the coolant level and it was below min, couldn't even see the coolant fluid level, so I rush to buy some coolant fluid to top it off, now the coolant level drops again, nowhere near the min level when engine is cold, and can't see the fluid in reservoir when engine is on. Checked for leaks and can't find that blue fluid anywhere on the ground, I've had a JB4 tuner installed when I got the car last November, am worried the fluid is going somewhere else in the car!! Could it be leaking coolant into the oil reservoir? Or into the radiator? What troubleshooting can I do next?