^^^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.
The soil pipe runs internally.
Its possible that it might be blocked as my dumb mate was tiling a couple of weeks ago and has blocked the sink in the bathroom(still goes down but slowly)and also a load of crap probably went down the bath plug knowing him.The bath drains quickly though.
There is definitely no water on any of the pipes or dripping unless its under the floorboards.