The letter said this
In preparation for the processing your application, we require additional documents/information to assess your eligibility and admissibility to Canada
In order to avoid delays in the processing of your application you must only send one submission in response to this request. That submission should contain all requested documents. Alternatively, if you are unable to provide any or all of the requested documents/information, please explain why they are not available, in the same submission where you submit all of the documents that you have prepared/that were requested.
If you have not yet submitted the following documents, you must submit them to this office:
Police Certificate(s)
The principal applicant (and all family members 18 years or older) is required to submit a police certificate(s) from the: • The country where you have spent most of your adult life since the age of 18; For instructions on how to obtain the correct police certificate required, you must visit
http://www.cic.gc.ca/english/information/security/police-cert/index.asp
If you have already submitted police certificates as noted above please disregard. If you are unable to comply please forward a written explanation.
I sent everything together in one big binder so it's unlikely that it fell out or something. Lots of people seemed to receive this letter within a few days of each other in april, i remember seeing an influx of posts about it...