I assume you were guilty of possession and were offered the possibility of a caution, which is an admission of guilt. The CRB check may or may not reveal your caution at this stage but any subsequent checks will reveal the existence of a caution, some employers require an annual CRB check.
Do you know you will be dismissed for this, presumably when you made your application you did not have this caution so cannot be accused of obtaining pecuniary advantage by deception at application, but you may be if you do not now disclose the caution.