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 & switch off Spotlight Suggestions & Bing Web Searches], but nothing has changed.
How can I find out what is downloading the data and how to stop it?
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Please help.