What exactly was remodeled? Could be something as simple to fix as a wax ring that didn't seal when the toilet was reset or failure to caulk around shower faucet to an improperly installed tile shower liner. Can you tell what fixture is in the area the leak is occurring?
I agree with Plumb26 on this ...we need a bit more info. did the bath remodel include relocating or reworking a shower or tub?...a loose slip joint could also be suspect.
to narrow down the options of where it might be coming from - locate common walls by measuring off major walls or outside walls on the lower level and then do the same upstairs
My guess its the vent pipe on the tub leaking. If the air vent was not properly sealed to the tub water that runs down the side and behind it will mimic a pipe leak every time you use the shower. Remove the two screws that hold the shut off handle for the tub drain, remove the drain itself and check to see if the fitting that is behind the tub that the screws fasten into has a washer or plumbers putty installed. If not put some plumbers putty between the fitting and the tub then re-assemble the drain handle and vent and you should be ok.
Another thing it could be, and again it can be a pretty simple fix is to check assuming you have a single faucet in shower or tub area is to remove the shower/tub handle and then remove the finish trim that surrounds the faucet itself. Quite often the foam gasket is missing or damaged that prevents water from leaking behind the trim and this allows water do drain behind the wall every time water gets onto the wall in that area. Just be sure when installing plumbers putty or caulk that you do not seal the bottom of the trim area. This will help allow water to leak out should some get behind the trim area.
Lastly, check the toilet wax ring. Push against the toilet with your knee, Can you rock the toilet? If so you need to remove it, find the reason for the toilet to rock, (uneven floor?) then replace the wax ring and reset the toilet to it does not move.