I just got the letter even though the last time I traded there was in 2012 and my cash and BTC balances were close to 0.
It's pretty annoying that my private information is being released to an ever widening circle of agencies even though I no longer have any interest in Mt. Gox.
If they were close to zero then you would still have an legal interest in the resolution of the bk case. If your balances were exactly zero then they would likely not be contacting you.