Without knowing what the symbols meant. they knew each symbol stood for a letter. There were four symbols written on the note, and only one suspect with four letters in their name. Therefore, the police arrested Ella. For further proof, the symbol ? was repeated twice in the same place where the letter "l" was repeated twice in Ella's name.
(I'm sorry if this was bad, I made this up myself and I've never really made up my own riddles :) )