For performance reasons autocomplete does not hide data from assets that are not visible to users - it shows all users the same completions. This means that care must be taken when configuring auto completion to avoid exposing sensitive data.
There are two strategies for avoiding the exposure of sensitive data. The first is to disable auto completion entirely. The second is to make sure that the "Auto complete" and "Include in keyword searches" options are turned off for all attributes that may contain sensitive data.