I setup a Pipeline job using a Pipeline script written in the job (not in SCM) and enabled SCM polling as a Build Trigger.
Looking through jenkins.log doesn't provide much information either, as this is the only entry around the same time as the SYSTEM user updates the config.xml
Jan 17, 2017 2:17:24 PM hudson.triggers.SCMTrigger$Runner run
INFO: SCM changes detected in unified-trunk. Triggering #84