Has the dealer got the service book for the car, also check the tax disc / mot cert (if it has been through one), these would have the mileage on them.
If they have "lost" the service book I would be sceptical. A car that is only 3 years old, with so few miles - I would expect the service book to be with the manual.
I believe the service interval for the corsa is 20 thousand miles (I may be wrong) so theoretically there may not have been any servicing done, but if they dont have the book, then there is something odd going on!!