How to search for characters displayed as octal codes in the buffer?

[ndame]

Sometimes I have unprintable characters in the buffer which display with an 
octal code (e.g. \232). instead of a character (text copied from elsewhere, 
etc.) How can I find these without knowing the exact codes?

I'd like to find all such places in the buffer where a character appears with 
an octal code, but I have UTF characters too in the buffer which display 
properly, so using nonascii is not the answer.