I am at the tail end of tiling my basement floor and undercut the existing trim on the bathroom door. I started noticing a sewage smell and pinpointed it as being strongest at this point. This is about 1' from the toilet, and the concrete seemed to stop here, so I exposed a small air pocket by cutting into the trim.
What could be the problem here and how can I address it? Would I call a plumber to remove the toilet and have a look?