I would have said that the letters were illegible,but not the notes that contain letters and numbers,which are legible. It's what they denote,rather than their illegibility ?
Illegible is when you can't make out the words in a letter,such as I was referring to.
We can all make out the letters and numbers as they're plain enough to see ( legible ) but what we don't know are their origins or what they represent. These aren't the ones which are being investigated.