Memcache is currently not logging anything to /var/log/memcached.log, since on Jessie the systemd unit is not configured to read any of /etc/default/memcached and /etc/memcached.conf.
A possible solution is to force memcache to use the -v parameter, and redirect stdout to a specific file via rsyslog rule. The man says:
-v Be verbose during the event loop; print out errors and warnings. -vv Be even more verbose; same as -v but also print client commands and responses. -vvv Be extremely verbose; same of the above and also print internal state transitions.
-v controls verbosity to STDOUT/STDERR. Multiple -v's increase verbosity. A single one prints extra startup information, and multiple will print increasingly verbose information about requests hitting memcached. If you're curious to see if a test script is doing what you expect it to, running memcached in the foreground with a few verbose switches is a good idea.
From a quick test in deployment-prep -v seems very low volume, and it shouldn't degrade performances, meanwhile -vv logs every request landing to memcache (definitely too high).