2003 Nissan Sentra Why is antifreeze being used from overfl

I checked and don't see any coolant leaking on ground. Checked oil, still clean, no antifreeze leaking into oil. Had new water pump, theromostat and
cap replaced 2-months ago, as car was overheating-o.k. since then, temp. gauge normal, have heat as well.

When checking fluids recently noticed that coolant level starting to go down a bit, not sure why, level is still above min. Picked up antifreeze to add to overflow-will keep an eye on same.

Can you tell me if there there may be a problem or problems brewing so I can have it checked now.

Thanks.