Subway door announcement mystery
The word you are looking for is 勿:


  請勿靠近車門 cing2 mat6 kaau3 gan6 ce1 mun4 (Jyutping romanization)


The usage of 請勿 ("Please don't") is considered formal or literary in Cantonese, so it's not typically heard in common speech, but you'll often see it on signs or in public announcements.

EDIT: Just wanted to also add that your deduction of the Mandarin is exactly correct.