404 Page Not Found --> assets in log files.

I have enabled errors to be logged and my site is running perfectly fine but i am always gettings these errors in my log files.

ERROR - 2015-01-22 15:27:42 –> 404 Page Not Found –> assets ERROR - 2015-01-22 15:44:22 –> 404 Page Not Found –> admin ERROR - 2015-01-22 16:22:03 –> 404 Page Not Found –> vtigercrm

Please guide me why these errors are been logged and how can i prevent it. Thanks