Two things I've found which don't show up in lsof output: ktrace.out
files associated with a process being traced but where the ktrace.out
file has been removed, and accounting files in /var/account or wherever.
Not sure if that will be the problem in your case though.
Aaaaaaaaah. That's interesting (ktrace), and might explain why the amount
of used space continued to go up despite the box seeming to be idle.
Anyway: The fsck did reclaim the space I expected.
Sorry to bring this to stable but I was worried something was wrong; The
box had been up for months with no apparent problems.