What was found
"MAIN CARGO FIRE" MESSAGE ON EICAS ON LANDING ROLL OUT. EMER EVAC OFF THE END OF THE RUNWAY. EMERGENCY DECLAERED. SHGN 27NOV21 04:44 ROC U WHEN ARRIVING AT A/C A WALK AROUND WAS PERFORMED. FOUND SAND AND DEBRIS ON BELLY OF A/C FORWARD OF ACM INLETS, INSIDE ACM INLETS AND LOWER WING SURFACE INBOARD OF PYLONS. AIRCRAFT SYSTEMS OPERATED NORMALLY DURING MAINTENACE TAXI AND ENGINE RUNS. REF MM05-51-56-2 SMOKE OR FUMES IN CABIN MAINTENANCE PRACTICES. PERFORMED 2 MINUTE HIGH POWER RUNS PER JRH TO EPR OF THE DAY. BOTH ENGINES PERFORMED NOMINALLY WITH ACCEPTABLE PARAMETERS. REF MM21-00-01-3 SMOKE OR FUMES TROUBLESHOOTING, ENGINE RUNS & OPERATIONAL TESTS SHOWED NO SMOKE OR FUMES IN CABIN. BOTH PACK SYSTEMS OPERATED NOMINALLY. REPLACED HEPA FILTER AFTER ENGINE RUNS PRECAUTIONARY REF MM21-25-02-4. A/C OK TO CONTINUE.