[arcatek]

@a is probably an alias for @all (https://github.com/search?q=%40all&type=Users&ref=searchresu...).

[edit 1] @everybody seems to return every user

[edit 2] @everything seems to return ... everything.

[edit 3] @qdfhsdfjsdqjrekle seems to return everything. Ok. It's a bug. :>

[pearjuice]

No, they are specifically searching for content by @{username} - refer to the cheat sheet on the search page. @{non-existent-username-or-user-with-no-activity} triggers this behavior.