There are several conclusions I could draw:
a) The maintenance guy looked in on the situation and kindly took care of it.
b) The neighbors did something perilous and swung over to my porch to take care of it.
c) The cats decided to play with their own excrement and knocked it off the porch.
d) Really, really high winds swept it off my porch.
e) I cleaned it up myself and have entirely forgotten doing so.
f) Somewhere out there is a superhero who flies around, dealing with unwanted cat crap.
I'd really like for it to be that last one.