Hi Carlo,
I looked into it and this stems from what I would call a rather faulty configuration of the memcache daemon in RHEL/CentOS 7.
The way to fix it is:
edit /etc/sysconfig/memcached and add:
OPTIONS=" -P/var/run/memcached/memcached.pid"
If you already have the OPTIONS var there, just add "-P/var/run/memcached/memcached.pid" in the end of it.
# mkdir /var/run/memcached
# chown memcached /var/run/memcached/
# service memcached restart
Then make sure you see /var/run/memcached/memcached.pid with the correct PID.
This way, monit should detect that it is correctly running.