OSX Yosemite uses over 500mb daily, without me using it. And only the total data received shown in Activity Monitor - no detail in specific process Almost every day, about 560mb of data is received and I'm not downloading anything. When I check in the Activity Monitor, it shows the 560mb usage, but in the list of processes, there is hardly any usage at all. If I sort by Rcvd Bytes, the most used by any given process is something like 2.7mb, and the total amount is less than 5mb (if looking at the details for the processes in the list)... I cannot find what process is using the data! Highly frustrating seeing that data is massively expensive in this country (South Africa) and I am using a wifi router with my 3G contract, and within a couple days, I've exceeded the limit.
I have seen a similar post and there was a process called nsurlsessiond which was addressed. In my case, this process name does not appear at all. I've followed the advice that was given in that post otherwise [like for Spotlight - Prefs > Spotlight > Search Results &amp; switch off Spotlight Suggestions &amp; Bing Web Searches], but nothing has changed.
How can I find out what is downloading the data and how to stop it?

Please help.
I don't think that the total displayed in Activity Monitor is the total you think it is. I think it is maybe just the last online action a process had. I noticed that while watching softwareupdate download data.



Just to be sure: 
Is automatic software update disabled? See System Preferences, AppStore. Also, do you have anything set to sync itself? iCloud (Photos and Co.?), AppStore, iTunes, stuff like that? 
For example, iTunes has its own "update settings" (see Preferences, last tab).

You can check in AppStore's "Updates" tab to see the recently installed updates. 



Do you have control over the router? Maybe it has logs with timestamps so you could see if any other device used it or what else happened! 
Because you said that the Internet is expensive, it could be that someone else is using it. 

I assume you have the WiFi secured and it is not public? Use the strongest encryption your router offers and I would change the password, just to be sure. 



May be it's the software that comes with the wifi router / 3G - have you installed anything for that?

Is this happening since beginning or started it later?

Also, you could ask your provider for logs or more information about this traffic.

Good Luck!