If the drain assembly is a trip and waste, I would remove overflow plate and pull the assembly out and check to see if soap scum and hair has built up on the cylinder that slides up and down to seal off the shoe to tub.
This sometimes will happen and the gunk will start to sour and emit smells from the overflow of tub.
It's a possibility, so like goodrich says, it can be a trap sucked dry from lack of a vent too.
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Steve @ Dunbar Plumbing
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