^^^What he said.
You need to check the connection from the bath waste into the soil pipe
Does the soil pipe run internally or externally?
If internal, maybe the soil pipe is partially blocked and the bath water is backing up and leaking out of a joint somewhere.
Even though you can't see any water you need to feel the pipes are dry, as sometimes the water can travel along the outside without seeing it.
If I were you, I'd do as much detective work as poss, as plumbers rates will be horrendous especially today.