Have O2 travel and even though mobile roaming and mobilr data switched off with no calls or texts made O2 travel is being triggered. Am in country on o2 travel list but have a vpn app which sometimes use which connects to country NOT on O2 travel list. Could thid be triggering data . Only using wifi.
What phone do you have, and do you have WiFi Assist turned on, as this will override the Mobile Data/ Roaming settings and will use the data even if for a minute.
Also the VPN could be falling back to Mobile Data, so check the App permissions, What VPN App is it?