ordering dungbombs

[Ms Mo Me]

No: HPFGUIDX 93387

This has always bugged me...

In Oop, HP goes off early on a Saturday morning to
mail a letter to Sirius.  This is the same morning,
when, at the owl post, he sees the horse fly out of
the forest.
Anyway, as he is going to the owlry, Mrs.Norris, the
cat, sees him and goes off to warn Filtch.  Then,
Flitch shows up and says he's been tipped off that
someone is ordering dungbombs.

Now, I don't think this is just a coincidental part of
the story since it is made rather prominant and left
open.  However, I have never been able to know what to
make of it.

My recent theory is that there is someone else in the
owlry besides Harry, Cho and Filch (possibly in an
invisibility cloak???) that Mrs. Norris saw. 

I don't really know.

Does anyone else have any theories as to what this may
mean, or it's significance to the plot?