There are quite a few areas of the house with dried blood drops
around. A friend feeding the cats on the last day before I got home
noticed them in one area. At least that means it didn't come from Espy
since he was staying at the vet. But it looks like either Marlo or
Nipsy (or the cat sitter!) was bleeding somewhat. There is only one
big drop, around the size of a dime, but quite a few pea sized drops.

Marlo did have what appeared to be a small scab on one ear so I'm
guessing that was it, but none of them have ever fought enough to
bleed. I'm also surprised that a scratch on an ear could bleed that
much and in several places (upstairs and down). Does it make sense
that the tip of an ear can bleed that much?

I haven't found any evidence on the cats of bleeding other than that
small scab, and even that is gone now.