1. A file creation method, comprising: providing a time-dependent database of reference data, the reference data comprising identities of wireless communication devices, reference locations associated with sightings of said devices, and times and dates associated with the sightings; creating file content at a creation location, which is unknown, at a known creation time, using a portable device; obtaining data identifying wireless communication devices detected by the portable device at the creation location at the known creation time; associating the data and creation time with the file content; and analysing the data identifying the wireless communication devices detected by the portable device to determine the creation location using the reference data in the time-dependent database, wherein the creation location is determined based on the reference data for the creation location and wherein reference data comprising the identities of wireless communication devices which appeared at the creation location later than the creation time are filtered out.