The Prius hybrid includes two separate cooling systems. Their roles are very different, as are servicing procedures. Engine as viewed from above. Note that the inverter reservoir is central and prominent while the engine access...
The Prius uses an independent cooling system to cool the inverter and Motor/Generator. In warm weather, problems with the inverter cooling system are fairly common in both the Gen 1 Prius (2001-2003), and Gen 2 Prius (2004-2009).