Sylday is right. I am British and live in Belgium. When I travel by car or train to The Netherlands, Germany, Luxembourg or France, I don't take my passport with me (I have a Belgian Carte de S�jour) because these countries, like Belgium, are Shengen signatories.
If I fly, I always have my passport because the airlines insist on it for identity (security) purposes because British citizens do not (yet) have a National Identity Card.
What is irritating with these passport questions is that the poster often gives no nationality or point of origin for the trip.