Do you see any smoke coming from your exhaust?
Do you smell a sweet smell inside the X5's cabin?
did the person/people repairing the thermostat refill according to proper procedure, OR did they perhaps capture an air bubble in the system?
Do you need to run the heater in your X5 much? If not perhaps the coolant was NOT in the heater core until recently you turned on the heater therefore bringing fluid into the heater core and therefore lowering the fluid level in tank reservoir as it was now filled with air. And sometimes this can happen by accident even to good mechanics. Coolant bleeding can take a few tries to get all air out of system
keep filling up with coolant in mean time. Perhaps it was just an air bubble in the system.

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I swear, my cars are like a girlfriend.
Sometimes its a rough ride, sometimes its smooth motorin'.
Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
BUT at the end of the day, she loves it when I am inside her.

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'91 850i
'05 X5 4.4i
'09 Clubman S