If one goes simply by her instructions to the students who will be doing the searching, she is looking for (and confiscating) food items and other forms of contraband from parcels.

It is, of course, far more sinister: she is reading all post, or delegating the reading thereof, and searching for sedition — or things she might twist into becoming such. I have overheard several things that cause me to think owls are first being redirected to MLE, as well, so that they are searched twice.

If you are expecting any post that you would rather no-one else in the castle see, let me know: parcels and letters addressed to staff are reserved for Hooch or Milland to open, and I doubt Hooch would mind being told that she could skip a day.