Check what is in the file
Open the preview and compare it with the layer list in a vector editor.
A background rectangle is geometry; a raster background is pixels. The checker identifies embedded raster content but does not decide which part you intended.
Make a safe correction
- Save a copy.
- Select and hide the suspected rectangle in your editor to see what it changes.
- Delete it only if it is unwanted geometry, or edit and retrace the raster source.