Normally, Ido does not include hidden buffers (whose names start with a space) and hidden files and directories (whose names start with .) in the list of possible completions. However, if the substring you enter does not match any of the visible buffers or files, Ido will automatically look for completions among the hidden buffers or files.
You can toggle the display of hidden buffers and files with C-a
(ido-toggle-ignore).