ORIGINAL from http://www.open.gov.uk/customs/discscan.htm
A traveller was recently stopped at Waterloo after arriving from Paris with a laptop computer. The computer hard disc was scanned for potential indecent or obscene images.
Just as Customs regularly intercept and examine baggage, items of mail, courier despatched material, and consignments of freight which might contain indecent or obscene material or other prohibited items we also examine the content of computer files. For obvious reasons we do not normally comment on operational methodologies or technical equipment.
Officers may examine laptop computers because experience has shown that these are being used to smuggle appalling paedophile images. These "scans" are done in the presence of the passenger. We would be failing the public if we were to ignore the scope for smuggling paedophile and other obscene material by this means.
Officers routinely encounter all manner of people and goods in their work, and, as you will appreciate, cannot be expected to be experts in every field. However, technical staff are available to assist where necessary.