It could be that water got outside of the tub when taking a shower and 漏水了.
Can not 找前OWNER.
Could also 漏水 from the walls around the bath tub. This is a common problem if the builder just use common dry wall behind the ceramic tiles. If this is the prolem, the repaire job will not be a small one.
Don't worry.
1) if the water smells, it may be the toilet...