I've spent hours today with THREE trying to resolve this. I was in Italy recently and this happened with both iPhones on my account: minuscule amounts of data are shown as being used on my account (total 6.6KB divided over 4 occasions) even though roaming is switched off. This caused the daily Go Roam charge to kick in.
THREE customer support is adamant that there is no problem, the bill is correct, and can't say what the data use was for 'security reasons'. At one point I was told that I had joined a 'maritime network' that wasn't included in Go Roam which, of course, doesn't explain the problem (and is just an attempt to confuse me - or, more likely that they are confused).
I suspect this is some kind of handshaking with the roaming network that is not being properly processed by THREE's billing platform. After some to-ing and fro-ing I was offered a 'goodwill' refund, but having read about this elsewhere during the hours of sitting on 'Chat', I've opted for a deadlock letter and have referred the case to the Ombudsman/ADR on the basis that I don't want to go though this every time I travel abroad.
It should not be this difficult to get a company to stick to its own terms and conditions so that we know where we stand when using (or rather not using) roaming services.
