I have a plan! It will take some imagination though. If you have some area in that bathroom as I suspect that is tiled, then it might be that the tiler poured something down the toilet.
1. Get yourself a cloth bag. It should be made from rough material. Pour some gavel in it or fish tank rock if you should happen to have that on hand. The bag will need to be sized so that it will slide smoothly up and down the pipe. Tie a rope securely around the opening of the bag. Put it in the pipe and raise it up and down to give it some scrubbing action. You might want to go to ace hardware and buy a 3 or 4 foot section of pipe that matches what you have in your home and use that for sizing and practice. This is how my brother cleans his chimney.
2. This one might be a little dicier. Create a plug out of a rag or old shirt, preferably pure cotton. Tie the rope around it and force it into the pipe. You will need a stick of some kind to push it down. Pour some water in and then pour some pool acid or muriatic acid in. If it starts bubbling wildly you will know it was from tile work. be very careful of the fumes. The acid will eat the rag up if it somehow gets away....well hopefully it will.