Moondog google: ["Sept. 18, 1931" China] to get your answer. Or see this link on the
Mukden Incident -- the date is an increasingly sensitive flashpoint for the neo-nationalists. It would appear that word has come down not to allow a repeat of incidents form earlier this year.
To the OP, in my years living in China I’ve never had ID demanded by police except when pulled over for traffic violations in which case all they have wanted was my drivers license and never asked for any other ID (e.g., passport, residence permit).