If you ever wondered why, despite all your efforts and Google-jistsu, you still couldn't get W3TC to detect your local memcached install, well. You haven't googled enough 😉

The hat trick is to swap 127.0.0.1 for localhost in /etc/memcached.conf:

# Specify which IP address to listen on. The default is to listen on all IP addresses
# This parameter is one of the only security measures that memcached has, so make sure
# it's listening on a firewalled interface.
-l 127.0.0.1    # <=== change this to: -l localhost
# TA-DAAAA

It took me some three days to eventually figure it out. But hey, I spend most of my life on a dishwasher. What do I know about internets?