Duke knew in his heart that the Crawford Clan must have sensed something; they must have been aware of Bell's death and sensed the presence of the curse on Duke's side at that fleeting moment.
They likely knew the exact location, albeit briefly, but it was uncertain if they could determine Duke's position in that instant.
Harbor City, just this one location, was enough for the Crawford Clan to ascertain a lot of information.
This place was Black Sail's territory, and also one of the locations of the Void Corridor's Teleportation Array.
The existence of the curse flickered briefly, appearing in Harbor City, making it nearly certain for the Crawford Clan that it was Black Sail who took out Bell.
Assuming the Crawford Clan could capture this fleeting information, otherwise, as long as the curse power on Duke could still be suppressed, the Crawford Clan wouldn't know who killed Bell for the time being.
