Not the usual cause!

Smell seems to be coming from near the waste pipe but there is no sign of any leaking liquid – could it just be gas that’s leaking? Is there a vent somewhere that could be blocked?

If there is a gap at the top edge of the waste pipe Joint, you would have a direct line (air) to the sewer.
But you would always see any liquid leaking if the gap is just at the top edge of the joint.

Post up a pic of the type of joint you have from the toilet to the waste pipe.

Is there a durgo valve that’s failed?