2005 Honda Accord. Coolant loss, after filling the reservoir twice. No leaks and oil normal. What could it be?

2005 Honda Accord. Coolant loss, after filling the reservoir twice. No leaks and oil normal. What could it be?

The reservoir is only supposed to be filled 1/2 way, if you fill it 1/2 the volume will boil out.

Could be bad head gasket or leaking water pump or leaking heater core, or leaking radiator cap, or a couple of things I can't think of right now. If you can't check these things yourself, have it done at a service station.