Fixed!

Wrong master.blade.php...

<header>
  <div class="container">
    <div class="row">
      @section('header')
    </div>
  </div>
</header>

<div id="sidebar" class="col-md-3">
  @section('sidebar')
</div>

<div id="content" class="col-md-9">
  @section('content')
</div>

Correct master.blade.php...

<header>
  <div class="container">
    <div class="row">
      @yield('header')
    </div>
  </div>
</header>

<div id="sidebar" class="col-md-3">
  @yield('sidebar')
</div>

<div id="content" class="col-md-9">
  @yield('content')
</div>

TL;DR - I should have been using @yield('something') instead of @section('something')