We've been advised by the management company's lawyer not to personally initiate contact with them or it could open us up to liability. So as much as I'd like to knock on their door to see what's going on, we've been told to stand down and only use the official communication channels with them (certified mail, etc.)

It is a younger couple that drives 2 different BMWs, so I doubt it is a similar situation as your neighbor. But no doubt something is going on with them.