{exists $path} is a plain file test: its content renders when the path is there, and the @else@ half when it is not. It is PHP's file_exists, so a directory counts as well.

The path is used exactly as given - none of PAD's application, data or _include lookups apply - so build it from the APP or DATA constant: a relative path is read from the server's working directory.

Use @else@ for the other half: {exists} does not cut an {else} of its own the way {if} and {auth} do. Written alone, without content, it shows nothing either way.