List what you know from the installation records and ordinary use, and clearly mark anything uncertain. Do not dismantle walls or operate fixtures just to discover their route.
What the observation means
A toilet, basin and shower may share a unit, but nearby position alone does not prove a connection. This uncertainty matters during fault planning.
A useful next step
Provide the unit model and any existing installation diagram. The engineer can assess the route safely. Avoid confidently excluding a fixture based on guesswork, and stop all known connected use if the system is not removing wastewater reliably.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.