Yes, a note and no matter what, she is not to be blamed because the mc did not notice that note before.... even if a talk will be better is at least more than she could do, and if you did take a moment to read it.... it was not even something she was allowed to do and yet she did, her crying and telling him that in a way is more than any can hope, some people are no good with words or talks, again is not the best way but saying that same argument that she left without explanation is a joke because while "The explanation was just a damm note in a shirt" it was still an explanation even if a bad one. Also, have in mind she wanted to tell the mc all those things in the beach but the mc joking around prevented her from wanting to ruin what may have been her last time together hence the note of someone that is really in love with him and not all people have the heart to say things in person, call that person coward if you want but not all are that brave.
I think at this point all of you are just trying to find something and looking at straws for a reason to keep hating her and I'm pretty sure after chapter 4 or 5 (in case driffty choose to wait til ch5) that photo of the bus stop is a way for her to tell him that she is going to give a full explanation of everything. And I'm pretty sure that will zip more mouths than chapter 3 already did.
Again you can think whatever you want since is a free world, so do I, however, try to understand that not all people do things the same as others, so don't try to justify that with facts .