src/Pure/Thy/sessions.scala

           val conditions =
             space_explode(',', options.string("condition")).
               filter(cond => Isabelle_System.getenv(cond) == "")
           if (conditions.isEmpty) true
           else {
-            progress.echo(
+            progress.warning(
               "Skipping theory " + name + "  (condition " + conditions.mkString(" ") + ")")
             false
           }
         }
       } yield name.theory