Is there any way to check on my own internet usage?

Question

I am really puzzled by my new satellite internet provider (Exede). I joine on 2/14 and by 2/16 they said I had exceeded (pun intended) my 10 gig monthly limit. I know I had not, so when I contacted them and had them check their details, they had my down for 4 gigs both that Sat (15th) and Sunday (16th) between 4PM and 9PM.
     a. I was not even near my iMac between 4:30 and 6:30, and was watching the olympics from7:30.
     b. I had not sent more than 5-6 emails, had not downloaded any videos, movies, etc.
     c. Their tech person said it was really strange even from their viewpoint, but could do little. He did give a super bonus (for free!) of two more gigs - never to be done again.
     d. Their tech person said they had complete confidence in their records and also believed something very strange was going on - like someone else or some virus was using my iMac. I live in a very rural area - thus the satellite connection try. I don't have any neighbors who use my wi-fi.
     e. Does the iMac send constant data on its own? Could it be possible for a virus or some other program to be doing such strange, unauthorized activity? Is there a way to check what acutally went on two days ago?
     f. His only other solution is to unplug their modem when I am not using the computer. Sigh.
Today I rechecked my use of the 2 gigs of bonus they gave me. I have carefully monitored my usage.
a. Yesterday I sent 9 emails, got about 20; watched about 5 minutes of a youtube video; and sent a 4.7 imovie enclosure via email. Their recorded me as using 5 gigs!!
b. Today (it is still am when I write this), I checked my email - 20+ emails, sent 10 (no enclosures) and watched exactly 4:30 minutes of Fcebook videos. Exede added 2 more gigs!!
I cannot figure out how this is happening. I guess I will have to cancel the service with only a week of being a user - unless someone can help me find the mysterious culprit, be it an unknown meighbor, a mysterious code in my iMac or ???

Answer 1

Any client on a network, including wireless clients you don't know about, could be consuming bandwidth. Wi-Fi networks must be protected with WPA2 security. If yours isn't, change the security setting on the access point. If it's already set that way, change the password. Make it a string of at least 8 random characters, including digits and upper- and lower-case letters. The password is saved in the keychain and you don't have to be able remember it; in fact, you shouldn't be able to remember it.
You make these changes in AirPort Utility, if the access point is an Apple device, or in a web browser otherwise.
If the network or the password was insecure, and bandwidth use returns to normal after securing it, you can be pretty sure the problem was caused by intruders.
But intrusion may not be the issue. Most often BitTorrent, iCloud, Dropbox, or some other cloud-data application is involved in cases of mysterious bandwidth use by a Mac. If you use iCloud, uncheck at least Photo Stream and Documents & Data in its preference pane and see whether there's any change. If you use third-party network backup or file-sync software, disable that. If you use a torrent client, remove it.
Otherwise, if you're running OS X 10.9 or later, you can see which processes are most active on the network.
Launch the Activity Monitor application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)  
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Activity Monitor in the icon grid.
Select All Processes from the View menu, if not already selected, and also select the Network tab.
Click the heading of the Sent Bytes column in the process table to sort the entries by bandwidth usage. You may have to click it twice to get the highest value at the top. What is it, and what is the process? Repeat with the Rcvd Bytes column.