I used this tutorial to integrate bootstrap in my project:
https://laravel-news.com/2015/10/setup-bootstrap-sass-with-laravel-elixir/
This places an app.css
I had the same problem with fresh install of laravel 5.2. What I did is just inspected the request path in browser which was :
../build/fonts/bootstrap/glyphicons-halflings-regular.woff2
so I just added to gulp.js this :
mix.copy('node_modules/bootstrap-sass/assets/fonts/bootstrap/','public/build/fonts/bootstrap');
and afterwards in terminal :
gulp
That did the trick for me!