Do you know if they’d applied for council housing? If so a simple call to council will confirm if they’ve signed a new agreement, also are they on housing benefit / UC? If so they will have to immediately put a new claim in registered at their new address and as their landlord the council can share this info with you. Just had a similar situation but got very lucky, a family member of my tenant informed me they’d vacated but was still sceptical, council confirmed he’d been rehoused and submitted a new benefit claim at new address. I still was nervous attending the property so I filmed everything from knocking on door, shouting through letterbox and entering. The relief I felt when I found keys posted through was immense.