For starters, I wouldnt eat anything left in your house. If it is a deranged adult, their warning you of getting sick makes it sound like they may have more sinister intentions than cold wind for you. But, sick isnt assaulted or robbed, so I'd say food/medicine/toiletries.

More likely though, this is a child. They wrote letters backwards, which some children (myself included) do, and the eye at the bottom could be a childs drawing. "Are you ready" could be are you ready for bed, or that they were writing whatever they were hearing. It's a week day, you said it was placed there between noon and 7pm, school ends between that time. Do any of your neighbours have children that may have wandered over?

To be safe, I personally would change out any opened perishable items, and/or wash bedding. I'd also call a non emergency helpline to see if someone could come sit outside your door in a squad car over night. If you live alone, it might be a good night to have company from some stronger looking dudes.