The Requirements Traceability Matrix (RTM) is the best document for the PM to check when a stakeholder believes a key functionality might be missing. The RTM links requirements # design/work items # test cases # deliverables/acceptance, allowing the PM to confirm whether the functionality is already included and where it is implemented and validated. This aligns to CompTIA Project+ objectives around maintaining project artifacts and using tools that ensure requirements are tracked through delivery and verification (i.e., ensuring what was requested is what gets built and tested).
A detailed scope statement can confirm what is in/out at a high level, but it typically won't provide the same end-to-end mapping to implementation and testing evidence that gives a PM confidence about a specific feature's presence. A task board shows current work status, not requirement coverage. A risk register is for threats/opportunities and responses, not for proving whether functionality exists in scope and has been built
/tested.
So, to avoid unnecessary change requests and to respond with evidence, the PM most likely checked the RTM to confirm the feature is already included and traceable to completed or planned work and validation.