A B757 Right Hydraulic system lost fluid during flight so an emergency was declared and the flight diverted. The APU was not started after landing because of contamination concerns and hydraulic was subsequently discovered in the APU cavity.

Narrative

During cruise; the right hydraulic quantity reservoir light and EICAS message 'RT HYD QTY LOW' came on. Checked hydraulic quantity; read .49 and slowly decreasing. [I] checked QRH. Only response was 'crew awareness.' Casually checked the 'HYDRAULIC SYS PRESS (R only)' checklist for guidance since the other checklist was basically worthless. Nice to know things lead us to more thoughtful analysis of our situation; especially the caution 'careful consideration should be given after the loss of a hydraulic system. Subsequent loss of a second system may create a more hazardous situation.' Since hydraulic quantity continued to decrease we decided to divert to a Maintenance Station. During the descent into the divert airport we decided to declare an emergency just to err on the side of safety. After an uneventful landing and taxi to the gate we decided to not start the APU since we did not know where the leaked fluid might have been. Post flight right hydraulic quantity was .22 and inspection revealed hydraulic fluid had flowed into the APU cavity. [It] could have been problem if we had opted to start the APU.