It doesn't matter.But you can inform them that they do not live there, merely send the unopened letter back stating as much. Whether they take notice or not isn't your issue.
It is only an offence if you INTEND to act to their detriment.
If you open it so that you can inform the relevant authorities, there is no offence.
But to try (probably in vain) and get the thread back on-topic....
If I had a forwarding address, I would be minded to call them and pass it on!