Classic Timing Analyzer constraints file

I am running the classic timing analyzer and I am having a problem figuring out where it's reading the constraints.

I have an sdc file, which seems is not read, because commenting out all the entries does not seem to change anything. The qsf does not include any timing constraints.

Which file then has the constraints?

the command line is quartus_tan

--read_settings_files=on --write_settings_files=off fpga

Thanks