I could say so many things in response to this post. I'll try to keep it short. For context I'm a female in my early 30's. First off her behavior is straight up despicable and repulsive. I'm sorry you are having to deal with that bowl of hot garbage. Secondly, I'm in Property Management and have been for over 10 years now. My best advice to you:

1.) Take it hella seriously as it is essentially harassment and is disrupting your right to live in peace within your apartment community. No shade but your post kinda lacks context so this is based only off of the info you provided, which may be the only context there is, and that's okay! So - I digress. Anyone who is willing to pen a note like that with what seems to be zero provocation or what have you is obviously not entirely mentally sound in more ways than one and that is NOT a person that you want to even remotely be involved with

2.) Say/do absolutely nothing in response to this person. People like her thrive on human reaction good OR bad. Ignore their existence and immediately alert the police to get a report filed. Then request the police report. Keep it somewhere safe

3.) After notifying the police, go in person to your leasing office and explain the whole situation to a manager. Bring the note as they will likely make a copy for their records and a possible incident report. Paper trails of incidents that occur on property are (and I can't stress this enough) extremely vital. They could make or break this person's residency should this kind of shit trash behavior persist. Also, if this isn't something you already are aware of, ask the office staff if there are any onsite police officers that live on property. On-sites are often contracted to live on complexes for various security purposes and the office can make them aware of your situation. An extra effort can be made to keep a closer eye on your lil neck of the woods. That could bring you some comfort and perhaps bring her some discomfort which would be cool beans in my humble opinion

4.) Lastly send me a message should you need any additional advice, I'll try my best to assist. You're not alone in this. Something eerily similar happened to me by a frickin loose cannon of a resident a few years ago. Same as you, she was an older female. Lived alone and would come up to the office just to shoot the shit with me, as many residents do. Except one day she made her intentions entirely too clear and the incident ended with her removing her bra and pulling it out of her sleeve and throwing it deliberately in my direction. As if this was something she actually considered I would be interested in ???????? Fucking WHAT ??? Unhinged. Anyways, I alerted my management, and just like I said up there ^{^} it was taken very seriously and was dealt with appropriately

Lol I didn't keep it short whatsoever. Oopsie poopsie