My point was, there is no data usage showing on mobile data or with the ISP that the wifi is connected to. The usage seems to have been caused by data being sent between the two S4's when they were both connected to the wifi at work.
I have an app installed which keeps rack of both wifi and mobile data (the android app doesn't give any stats for wifi).
The 24-hour period in question shows 0.8 Gb between 1-2pm and 3.0 GB between 4-5pm and no other wifi usage of significance on that day, and no mobile data.
Despite 3.8 GB having been recorded as entering or leaving my phone via wifi, there is no evidence of this data on the ISP's logs - hence the inference that it was between 1 S4 and the other. The apps shown as using this data are OS services (3.17GB) and DirectShareManager (600 MB)