My daughters 2005 nissan sentras antifreeze reservoir has to be refilled every 4 days I don't see any fluid leaking.What would cause this?

A leak in a pressue hose under some where, small my guess.
Need to have it taken in for a test of the system.

Definitely have a pressure test done on the cooling system. It could also be an internal leak (like a faulty head gasket) which would be why you're not seeing anything on the ground.

Pressure test it or add leak detecting dye to the cooling system and run it for another four days and then inspect for traces of dye.
Are you having any other symptoms? Running poorly or the engine oil looks like a chocolate shake instead of oil?