Not sure if it’s there by default. As i like to have separate log files, it might be possible that i added the rsyslog config myself. Tbh, i don’t remember 
The messages should also be available in syslog.
If you want a separate log file:
Check if /etc/rsyslog.d/slapd.conf exists.
If not then create it with the following content:
if $programname == 'slapd' then /var/log/slapd.log
Then start rsyslog
Also increase verbosity of slapd according to How-To: Enable Logging in LDAP
Then try to start slapd and check your logs