I'm seeing the following error message any time I try to use Drush to install a module:

    This codebase is assembled with Composer instead of Drush. Use `composer 
    update` and `composer require` instead of `drush pm-updatecode` and `drush pm-
    download`. You may override this error by using the --pm-force option.

I'd like to find out why Drush is generating this error message, as the website was not built using Composer (though there are other Composer-based Drupal project on the same web server).

FWIW, using the --pm-force option works just fine. This problem is more an annoyance than anything, but I can't figure out what Drush is checking that causes the error to appear. This is a Drupal 7 site, if that makes any difference.