This morning after arriving at work, I noticed a slight gap between the end cap and the base of the fender mounting assembly around the left fork tube. Closer inspection revealed one of the 9/16" hex bolts in the base had worked its way loose and was rubbing the front tire's sidewall. I re-threaded the bolt through the cap and tightened it with a standard box-end wrench. The two-piece fender mount is nice and snug around the fork tube now with no air between the cap and the base. I checked the second bolt on the left mount and the two on the right side. All is well. The tire's sidewall is fine, too. Disaster averted. I'll live to ride another day.
Remember your regular "pre-flight" safety checks. Add to the list as necessary. New item for me.