50 Cent's staffer in charge of watching over his Connecticut mansion feared an alleged burglar might be armed and dangerous ... according to his 911 call.
Fiddy's house manager told the dispatcher a burglar was on the property going door-to-door -- it's a massive spread -- looking for a way inside. He said he didn't have eyes on the perp anymore, because he'd fled the home for his own safety.
The manager said the guy had a black bag ... he didn't know what was in it, but knew he had nothing to protect himself.
As we reported ... cops showed up shortly afterward and busted Elwin Joyce, who was wearing dark clothing and had a printout of directions to Fiddy's home. He's now facing 4 criminal charges, including 2nd degree burglary.