It sounds like someone has programmed the machine for check validation. I haven't worked with this series but there are manuals here:
OPERATION MANUAL
http://the-checkout-tech.com/manuals/cashregisters/Samsung_SPS-500_operator_manual.html
PROGRAMMING MANUAL
http://the-checkout-tech.com/manuals/cashregisters/Samsung_SPS-500_programming_manual.html
Registerman, May 2013