The following is from Draft 2 of the ACM Policy on Complaint Process Disclosure.
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2.2 Complaint dismissed prior to investigation
When complaints are filed without any evidence or anonymously such that no investigation is reasonably possible, the complaint will be dismissed without investigation and information disclosure will be treated as if no complaint has been filed (2.1).
will information about the complaint be retained by the ACM, in the event that there are additional complaints in the future? For example, a witness files a complaint but they don’t know the identity of the person targeted, and later on the person targeted files a complaint
To the best of my knowledge, ACM has no formal policy in place for this situation. As a matter of practice, though, ACM does retain the information about complaints for this purpose.
I understand the difficulty in investigating something without any leads to proceed with. Sometimes, it is not just an individual, but an event may inadvertently enable poor behavior by some attendees against others. How are these kinds of things collected and are there thresholds for particular kinds of actions required? For example, installing surveillance equipment may be called for to record public spaces that seem to be problematic to support later investigations. Encouraging people that have been affected to report even though they have nothing from which to form an investigation are all important factors to making events safe spaces for everyone.