So, I noticed every month recently I am getting apparent data usage which is getting quite costly, so i decided to test this and leave my phone at home, my data usage is still accumulating. I ONLY use wifi so how is this possible?
Are you using an iPhone? It has a feature called wi-fi assist where it will use cellular data if the wi-fi signal isn't strong enough. Check out settings and cellular to see that it's off. Otherwise and overall also make sure that data is turned off then when you're not in wi-fi as background processes could also be taking up data
All smartphones use data in the background as Goran said. Sometimes to sync with your email accounts and social networking accounts to see if there is new mail or status updates, things of that nature. Unless you disable your data connection there is a chance that applications are using data in the background. You can also change in the settings that your apps only update in wi-fi. If your wi-fi connection becomes weak and data is on, your phone will seek a stronger connection which will be our network.