Once again, many thanks for your taking the time and trouble to assess the situation.

I ran Verify Disk under Disk Utility's First Aid with the following results:

Verifying volume “XYZ”
Checking file system
Performing live verification.
Checking Journaled HFS Plus volume.
Checking extents overflow file.
Checking catalog file.
Checking multi-linked files.
Checking catalog hierarchy.
Checking extended attributes file.
Checking volume bitmap.
Checking volume information.
The volume XYZ appears to be OK.

Under the circumstances (ie, no apparent issues requiring repair) I saw no point to running off Recovery HD in order to use the Repair Disk function to repair or rebuild the volume directory (and I don't even know if such is an option there, but I haven't looked).

As for performing a surface scan, I don't have a clue where I'd even start, and Pogue's Missing Manual doesn't even mention such.

Since my hard drive was replaced less than a year ago and since nothing truly wonky is happening, I think that I may take the 'wait and see' approach.

Time for a nice glass of Grenache/Shiraz/Mourvèdre, while I flip my latest batch from the primary to the secondary.