Toyota Highlander Hybrid 2010 Manual page 440

4-3. Do-it-yourself maintenance
Coolant
The coolant level is satisfactory if it is between the "FULL" and "LOW"
lines on the reservoir when the hybrid system is cold.
Engine coolant reservoir
440
Reservoir cap
"FULL"
"LOW"
If the level is on or below the
"LOW" line, add coolant up to the
"FULL" line.