How is QGIS opening a shapefile with no *.prj file?
QGIS has a good (in my opinion) mechanism for deciding what to do with input layers that have no coordinate system specification. You can choose one of three ways for QGIS to behave:
- Always prompt for the correct CRS
- Always assume that the current project CRS is correct for the input layer
- Always use a certain default CRS that you choose
You can set which of these three is used in the Settings window, under CRS.