This is an example of "spontaneous recovery".
Spontaneous recovery refers to a phenomenon that includes all of a sudden showing a conduct that was believed to be wiped out. This can apply to reactions that have been shaped through both established and operant conditioning. Spontaneous recovery can be characterized as the return of the conditioned reaction after a rest period or time of reduced reaction. In the event that the conditioned stimulus and unconditioned stimulus are never again related, termination will happen quickly after a spontaneous recovery.