All too common problem is a failed or mis-applied wax ring, or rotted closet flange, allowing water to infiltrate the subfloor. The leak will not present itself around the base of the toilet unless the closet flange is sealed to the finished floor, or if the leak flows quickly enough to flow from beneath the toilet. If your leak is flowing down the outside of the drain pipe, it may not be getting into the subfloor - that's good. Regardless, I would pull the toilet, inspect for wetness in and around the closet flange to identify the actual leak path, and repair as appropriate. I had to re-plumb both closet flanges in my house to bring them up to the level of the remodelled finish floor, thus allowing the was seal to remain water tight. What a hassle.