Students in Bangor, Gwynedd, are being urged to take extra care when using their passports and driving licences.
Many use the documents as formal identification but 21 passports, and "dozens" of driving licences have already been handed in to North Wales Police this year.
Police keep the passports for 24 hours before they are destroyed.
Of the 21 handed in, only one has been handed back to its rightful owner - who needed it for an imminent trip abroad.
"Every year in Bangor alone we probably get between 20 and 30 passports handed in," said western Superintendent Peter Newton.
Supt Newton added it was mostly students and young people who use passports and driving licences as identification.
Passports are kept by the police for 24 hours and then a corner is snipped off, which destroys the document, and they are sent back to the passport office.