Bandwidth by Day uses 115 MB without any visit user.

Check your servers logs or traffic analysis to see what files or resources are being requested. There’s really no other way to do it.

As above, check out awstats (if you have cpanel) to see where the traffic is coming from.

My guess: bot traffic.

Try implementing the Hackrepair.com bad bot list in htaccess to block them.

Or, go a step further and implement ithemes security and lock it down