The typical failure is the UIM itself, not the gasket. Inside the UIM is a hole where the EGR (exhaust gasses) pass through a tube. That tube heats the plastic UIM and the UIM over many cycles cracks into a pressurized coolant port that leads to the throttle body. The pressure of the cooling system them pushes coolant into the motor. The LIM will hold approximately 1/2 gallon or about 2 litres for our neighbors to the north aka OP.
At the point of the small amount missing, I would refill and keep an eye on the level. Some coolant can evaporate and a small amount over years is normal.
As others have pointed out, check for external leaks etc and if the UIM is suspected to be bad, replace it and the LIM gaskets at the same time. Or plug the coolant ports and have no future worries about the UIM failure.