tmux session lost in unknown pts, cause and possible solution?

Solution courtesy of the Webfaction-support:

As the process was still running, the issue was a deleted socket, possibly caused by a purged tmp-directory.

According to the tmux mapage:

If the socket is accidentally removed, the SIGUSR1 signal may be sent to the tmux server process to recreate it.

So sending the signal and attaching works:

killall -s SIGUSR1 tmux
tmux attach

Terminal absence is a sign of detached session. And all your tmux session names can be found thus:

ls $TMP/tmux-$(id -u) or ls /var/run/tmux/tmux-$(id -u)

— this is kinda distro-dependent. Almost distro-independent (and more hardcore) would be:

lsof -n -p 16709 -a -U

where 16709 is the PID of tmux in your listing.