The train is approaching? 火車未到站

Cantonese euphemisms/ idioms to remind our conversation partners to fix something about their appearance 

對方儀容的禮貌提醒:

火車未到站 "The train has not yet reached the station"

=Your fly is down

The zipper teeth are the rails and the pull tab is the train; the top stop is the station 

(BTW, I found an app that reminds you whether your barn door is open called "Noti-Fly")

留番做宵夜呀？ "Saving it for a midnight snack?" (Usually used as a question)

=You have food residue on your face/ lips

This one is self explanatory. 

(Someone could have used this with Ted Cruz during the debate) 

I wonder if you can say this if you have food stuck in your teeth? (I definitely need an app for that)  

I couldn't think of anymore. How do you say your hair is messy or you have a hanging booger? So I made some up (Thanks to SS for contributing): 

Made-up idiom #1

A: 今日全港學校停課呀？

B: 吓？

A: 八號風球呀？

B: 吓？

A: 打風呀?

B: 吓？

A: 你個頭呀！梳吓佢啦

Made-up idiom #2

A: 你果七個小矮人去咗邊呀？

B: 吓？

A: 你d礦工罷哂工？

B: 吓？

A: 冇哂礦工採礦?

B: 吓？

A: 你個鼻呀！整吓佢啦

禮貌提醒定係搏俾人打？

Do you know of any similar phrases?