Hi Shareef.
Thank you for an excellent question. If a risk that was not on the risk register occurs, this is what happens:-
1) It immediately becomes an issue and should be noted as such. This is because an issue is an actual occurrence on a project whereas a risk is an occurrence that may happen in the future. This also implies that a risk that was in the register can also become an issue if it occurs.
2) An issue that was not part of the risk register is not planned for therefore it means that you do not have a contingency for it. It, therefore, requires that you use the management reserve which necessitates a formal change request.
I hope that helps.
Best.