Why is my tub draining under my foundation?

When a concrete foundation is poured, the tub drain pipe is left accessible for the rough-in plumbing by installing a box and dirt to prevent concrete from being poured over the pipe. When the tub is set after the house is framed, the plumbers will emble the p-trap and connect the tub drain to the pipe which flows to the sewer system. It is possible that the p-trap has come loss or needs replaced. If this is the case, a section of the lower wall (sheetrock), where the faucet and drain is located, will have to be removed if an access is not there (some cities require the access for their code purposes). I would immediately contact a professional plumber and your insurance agent. Water undermines the integrity of the foundation and causes extensive repairs and depreciation of property.