Answer:
See Question 6 from a recently published FinCEN FAQ.
The relevant quote is:
The time to file a SAR starts when the organization, in the course of its review or on account of other factors, reaches the position in which it knows, or has reason to suspect, that the activity or transactions under review meets one or more of the definitions of suspicious activity.
They are not mandating a knee jerk reaction because one person thinks something is suspicious. They want you to investigate; the time frame for filing begins to run when your bank (hopefully some deliberative body) thinks the activity is suspicious.
First published on BankersOnline.com 03/01/04