It might help if you give a little detail about your car generation or made year, since there might be variation on power steering system.
Does the car manual or auto transmission? Is it share power steering fluid with clutch fluid, leak might occurred at clutch cylinder where you might did'n look at.
Edit: read the problem description again and found that you have fluid lost but don't found any leak, sorry. Keep the previous answer just in cast that might help others
Assume your system is pure hydraulic-based. Power loss in hydraulic system might occurred without any leak due to worn/leak internal system. Or it can be from not enough power to drive the pump.
Have you check the power pump belt?
Have you look for any unusual thing that might block the steering rod or wheel from moving?
Are there any unusual resistance to the engine (belt make noise) when you try to turn?
The power steering is not assisted at all (like your didn't run the engine) or it lost some power?
PS. sorry for my bad English