Did you check your text messages? Usually you get a "Welcome to our network" text any time you connect while roaming. With international roaming so expensive, it is very easy to rack up a giant bill just in background processes. If your mail and other apps started to sync on approach and during taxi, you could easily use many megabytes.

Last time I drove over to Canada I got the "Welcome to Rogers" text and didn't read beyond the $1/min call rate. Turns out the data charge was $10/MB. An hour later I got another text saying my roaming bill was up to $50 even though my phone was in my pocket, based only on background data.