One of the most likely culprits is the closet flange, a fitting located underneath the toilet. Experts recommend setting the flange flat on the bathroom floor or no more than a quarter-inch above it. A replacement should be in order if a large chunk of the flange has broken off. You can find one for about $5, but installation isn’t easy.
In relation to the flange, there’s also the matter of caulking. Most plumbers may not recommend caulking because it would be difficult to spot any future leaks, but some municipal codes deem it necessary to prevent bacteria from gnawing at the flange. Ask a Katy, TX plumber like Capital Care Plumbing about caulking first.