Yes due to bubble somewhere in the engine

And once the engine cools down, the coolant in the tank will be sucked into radiator.  That is why you have seen the empty tank couple times.  Again, even they refill, they did not follow the SOP.  They should have the cap open and keep adding coolant until the engine is hot and themostat is open, around 85-90 C.  Otherwise, the bubbles can not be eliminated completey.