The S2 has 16GB storage in total, but it's partitioned into a 2GB /data partition and an "internal sd card". This was a Gingerbread phone originally, so pre-dates unified storage, and as I recall Samsung kept the same partitioning when they upgraded to 4.x (removing it would wipe the phone, and an OTA which did that would annoy a lot of people).
So actually an S2 with 2GB "internal storage" sounds pretty plausible to me.
My puzzle is how it got down to 16MB free? It should have complained earlier than that.
Try clearing app caches, checking whether any apps are using a lot of data. App data use the same space as apps