netstat does parse /proc/net/[tcp|udp|tcp6|udp6] and then scans the /proc/$pid/-area to find matching processes, while fuser and lsof do "only" travel through the whole /proc/$pid/-area to find which processes are known for which sockets. Ultimately, those two different approaches may end in different results.
If there is no process assigned to the socket, there are two options:
- You're using lsof/fuser and there is no process allocating this socket,
but e.g. a kernel module is doing so. You need to find that module.
- you're running netstat as an user who may not access /proc/$pid/fd/.
For example, mounting a share via NFS does result in similar "listening" ports. So those ports will re-appear by mounting a NFS share and will disappear after unmounting the NFS share.