I saw this Xiaomi x Hello Kitty collab phone and I think it's perfect. From my understanding when UV light hits it the subdued Hello Kitty faces turn red/pink. Exposed sissies would be able to proudly show it off while secret sissies (that are into the thrill of getting caught) would always have it in the back of their mind if someone notices.
https://www.notebookcheck.net/fileadmin/Notebooks/News/_nc3/civi2hellokittyteaser.jpg
Wow, that's a great idea! I'd never considered symbols that would only activate under UV light
To expand on this..... For the wallpaper. Exposed sissies would have an out there wallpaper and icon theme pack that no doubt shows you're a sissy. Pink and feminine maybe but depends on the style of sissy you are. Eg. Sissy puppy might have something more themed at sucking on delicious "bone(r)s" and alpha pup cocks (like a yummy bad dragon fantasy cock), Sissy Maid maybe something more cleaning and serving, sissy whore something more about taking cocks, ABDL sissy will have more cutesy stuff etc.
Secret sissies definitely something subtle where you'd pay no mind if you glanced at it, but if you looked at it for more than 30 seconds or thought about what the wallpaper is visually saying you'd get it.