I found it!

[Robert Goodale]

Thanks to those who responded regrding the falboard marks.
I figured as much regarding fingernails, I guess I was just
trying to figure out "how".  It might be interesting someday
to cinemgraph the keyboard in use in duper-slow motion,
(i.e. 12 or 15 fps).  This might show how it all really
works.

P.S.  I found the missing sock- The CAT took it!!

Rob Goodale, RPT