Unfortunately you have left out some important information like:
- is this slab on grade or crawl-space
- interior or exterior doors,
- any changes recently, etc...
Clay soils have been known to expand and contract with changes in moisture content so that could be an issue.
If this house is built on a crawl, you may find that blocking was not placed between the joists where the walls land so that could be an issue.
I am also curious when you said, they are not where you have seen them before - have you had an ongoing issue?