The details of this moment are not fully documented in the public record—which colleague, what was said, what specifically caught their attention. The full account of that overheard conversation has not been published in the sources available. What is clear is that by September 2007, Cleveland Police had opened an inquiry into Anne Darwin. The September 2007 start date for the formal police investigation is established in court documents.
This timing matters enormously for the chronology of how the scheme collapsed. By September 2007, the Darwins were approaching the end of their Seaton Carew period. The family home would sell in October 2007. Anne would leave for Panama. The couple had been operating the scheme for more than five years and were in the process of executing the final phase of their exit. The police inquiry began while they were still in the middle of this transition.
We don’t know, from available evidence, whether the Darwins were aware that a police inquiry had started. If they were, it might explain the timing of John Darwin’s return to England and his decision to walk into the police station in December 2007—choosing to surface on his own terms, claiming amnesia, before the investigation reached a point where an arrest warrant would be issued and the terms of his return would be determined for him. If they weren’t aware, the timing of the return is harder to explain in strategic terms, and the accounts about missing their sons have to be taken at somewhat higher face value.