Is it possible that the cat brought a death bird or something and the blood stain is actually from that prey?? You see, I realized cats when they are thankful to a person, they leave strange gifts for them... I've found in my doorstep death birds, little mice, even a little snake once! It is their way of showing respect and appreciation I assume.

Also, cats disappear every once in a while for a few days, they are cats. I've had more cats than I can recall, all of them rescued from the streets. My sister would cry so hard whenever one didn't show up in a few days... then the cat would come back again and eat, enjoy being petted. Don't panic, cats behave like that!

I'm just trying to cheer you up a bit with other possible things that might have happened...
Good luck and I really hope the cat comes back!!