This is the plugin:
http://wordpress.org/extend/plugins/bookings-engine/
I've deleted it (Deactivate, then delete) several days back, and beginning April 11, 2013, I check the website I'm working on to build, and the menu styling will be messed up. I checked what happened, and that "Bookings Engine" plugin is back on the "installed plugins" section.
No, the web-host/server is not reverting to an old backup of the whole site, because the changes I made before the plugin installs itself automatically still is present (i.e. I wrote a post on April 11 (the plugin already deleted), and on April 12, the post is still there, but the plugin is also back).
I also tried checking the FTP, and manually deleted the plugin (bookings engine) folder on the plugin directory.
How do I track a hidden file/robot file that sneaks a plugin invasively?