I mended the quilt batting with tape as I was not finding it easy to join two edges by hand - either thread too tight or too loose. When I pinned it back together I noticed a chewed area right on the seam at the right-angle corner of the outside strip of the corner triangle, of all places, so I had to perform a little strip surgery on each side of the strip, which I did by hand. Lessons learned there!
I looked under the sofa and to my horror discovered that it contained several of those little felt catnip mice, half chewed, and a rather thick scattering of mouse poop, along with other bits of fuzz. I don't think the catnip mice were pooping, but were the real mice playing with the catnip mice? Hmmmm. Poor old cat has been gone for a few years.
Housecleaner evidently didn't move sofa ever (shudder) so I am now on a real spring cleaning of my own! There's another positive outcome.
I put two traps out, baited with PB and toast, and added a couple scraps of batting on top to attract their attention, but no bites yet.
Well, all's well that ends well.