What they reported was the drive by presence of armed militia men whom were reconnoitering the Mission to determine the defending force.
That was a bit unusual but not unheard of, Benghazi is frequented by cruising armed militia men of unknown loyalty, many are friendlies.
There is often random gunfire also.
Directed gunfire began at 09:38 hours. That’s when the tactical situation changed.
All of this is secondary to the essential question:
Why was the SST (Site Security Team) sent away at the end of August without any comparable security left on station?
The SST was paid for by DOD funds, so no excuse there, they were free to the Dept of State for one year with another year extension if requested. No request or review was made by State. The SST, to a man, had offered to stay deployed another three months, but were rejected and went home.
The lack of security, heavy weapons capable, is the defining reason for the attack.
BTW, all comments regarding why we didn’t relieve Benghazi with other rapid response forces fail to understand the mission-driven nature of all our military.
Any relieving force at Benghazi would have to be capable of night-vision and HRST:
http://www.combatindex.com/store/MCWP/Sample/GROUND_COMBAT_OPERATIONS/MCRP_3-11_4A.pdf
and have CH-47 (Phrogs) mission capable of insertion.
There were only three - company sized units available. Two were committed to relief of the embassies in Cairo and Khartoum and the other to the AfPak theater.