Forensics · 5 min
The disk is full and nothing is using it
What you were told
df says the filesystem is nearly full. du over the whole tree accounts for a fraction of it. Somebody deleted a large log file an hour ago and the space never came back.
The machine is beside you as somebody handed it over. There is no trace — you did not start these processes, and on a real machine you would not have one either.
Diagnose
Why has the space not come back?