Hi. Welcome to Just Answer.
Anytime the soil near a foundation begins to fall away or sink it is cause for concern. It could be ground setteling, water run off washing the soil away (either above or below ground) or even the beginnings of a sink hole.
The best course of action in cases like this is to have a soils engineer come out and have a look at it. There are tests they can do to determine the compaction of the soil and whether there are voids below the ground surface.
Although this can be a bit expensive it is money well spent considering the possible damage that can occure if the issue is not addressed.
If it is just soil setteling then usually just a little compaction work and back filling the spot will be fine. Anything more than that you will just have to see what the engineer has to say.
Please let me knoiw if you have additional concerns.