Nexus 3's 'Search' feature handles empty fields inconsistently. Most fields, when empty, don't constrain the search at all. They have pale, 'ghost' text that reads, 'Any', signifying this. When the Keyword field is empty, however, no results are returned, even though it also has the 'Any' ghost text. This behavior changes as soon as any other field has a value.
This threw me during testing, as I had added a number of format-specific fields to a search, deleted all the values and was now getting no results.