My neighbor leaves a single, specific passive-aggressive note every time it rains

Whenever it rains I leave my umbrella in the hallway to drip-dry. Every time, without fail, there is a handwritten note the next morning: 'Please remember hallways are shared spaces for ALL residents :)' The smiley face is doing heavy lifting. I have never seen this person. I believe they may only exist when it rains. I have started leaving a thank-you note in return.