All populations of wild animals fluctuate for a variety of reasons. When populations get too large, crowding causes competition, stress, disease, and parasites to become prevalent and the species crashes, sometimes to the point of temporary local extinction. In areas where humans have severely altered the habitat, there is often a long lag before individuals from the surrounding countryside can move in and reestablish the species. Where I live we have periodic outbreaks of distemper which completely wipes out the wild fox populations. After these outbreaks, it takes years for us to start seeing foxes again. It is possible that something caused your local mustelids to crash and it may be some time before they get reestablished. If the suburban development around you is too dense and there are no wildlife corridors, your local extinctions may become permanent.