As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LGW - FLT 127 - AFTER TAKEOFF, CREW RECEIVED THE EICAS MESSAGE R HYD QTY AND THE ASSOCIATED LIGHT. THE HYDRAULIC QUANTITY READ 0, BUT THE PRESSURE WAS NORMAL. FLIGHT RETURNED TO LGW AND LANED WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE HYDRAULIC QUANTITY MONITOR.