If Clearwell cannot search through the content, the item will auto-collect.

Clearwell Collections were designed to collect data if an item is not fully determined to contain the keywords.
The condition occurs when the Task fails to search the content of a message, or its attachments. 

Examples of items that are collected that do not match the filter criteria:

Items that do not fall within the Date Range filter:
When an item's sent date cannot be determined, such as email Draft items; those items will be auto-collected.

Items that do not fall within the Keyword or Phrase filter:
PC, File Share and SharePoint Collectors are unable to scan email items for subject or content.  All email items such as .EML or .MSG are auto-collected.

All Collection type collectors will not scan Encrypted items, Password protected items, Images or items that are within a container file, so these type items will be collected.
Examples of container files are ZIP, PST, NSF, OST, an attached email with an attachments... etc.

Exception Note:
Enterprise Vault (EV) collections utilize the EV index to search the attributes of each item, so the above conditions do not effect filtered EV Collection Tasks.