if you enable logwtmp option on your pptpd.conf, you should see on /var/log/messages
pptpd.conf
# TAG: logwtmp
# Use wtmp(5) to record client connections and disconnections.
#
logwtmp
/var/log/messages
Mar 13 19:45:23 pineapple pppd[25571]: No CHAP secret found for authenticating USERNAME
Mar 13 19:45:23 pineapple pppd[25571]: Peer riqa failed CHAP authentication
Mar 13 19:45:24 pineapple pptpd[25570]: CTRL: EOF or bad error reading ctrl packet length.
Mar 13 19:45:24 pineapple pptpd[25570]: CTRL: couldn't read packet header (exit)
Mar 13 19:45:24 pineapple pptpd[25570]: CTRL: CTRL read failed
Mar 13 19:45:24 pineapple pppd[25571]: Modem hangup
Mar 13 19:45:24 pineapple pppd[25571]: Connection terminated.
Mar 13 19:45:24 pineapple pppd[25571]: Exit.
Mar 13 19:45:24 pineapple pptpd[25570]: CTRL: Client w.x.y.z control connection finished