Your plumber is going in the right direction. Sewer gas is getting past the water seal of the trap or coming out elsewhere. More investigation in the area is necessary to find the leak that can be coming from anywhere in the area – an unsealed joint in the vent, a crack, etc. The techniques for finding a leak, smoke testing, etc. are beyond the ordinary. Ask the plumber to keep going until the smell disappears, that is both the art and science of the purpose of his expertise.