Disclaimer: I don't own KHR or any of its characters.
At first, when Chrome first notices the post it notes, she thinks that it's because the two can't communicate without insulting each other. Looking at the post it notes, it is actually a very reasonable deduction.
'The pineapples are running out. Get some.'
'Kufufu~ not until you stock up on the chicken, Skylark-kun.'
But when Chrome checks the fridge, it's always fully stocked and there are pineapples everywhere in the house; on the dining table, the kitchen counter and even the bedroom. Yet, the post it notes are still stuck on the fridge.
'Where's my pineapple salad?'
'What about my roasted chicken? You should really cook sometimes, Kyouya.'
'Hn.'
Chrome wonders why they don't just talk. In fact, she rarely sees them talk. When they are together, the normally more talkative Mukuro-sama stays absolutely silent, always hovering about Hibari-san but never once actually addressing him.
'I expect nicely cut up pineapples when I return.'
'Kufufu~ I forgot~ But I do want some chicken rice when I get back.'
'Buy some yourself.'
However, she starts to realize what exactly those post it notes mean, when Mukuro-sama is hospitalized after being tortured. She never sees Hibari-san visit Mukuro-san once but when she goes to their house to check up on Hibari san, she finds that the fridge's surface looks incredibly desolate without any post it notes stuck on it. Later, when she goes to visit Mukuro-sama, there is a basket of pineapples with a note attached to it.
'This is the only time I'll buy the pineapples. You owe me.'
Chrome's never seen Mukuro-sama laugh so much.
The next time she visits their house; there are two new notes on the fridge.
'I brought the groceries and even an extra pineapple~ Make dinner for me?'
'…Hn. Maybe.'
This time Chrome just smiles and leaves before they notice her.
~fin~