Problem: debug set to true in config/app.php, vendor/cakephp/debug_kit exists but debug kit toolbar not appearing in browser, debug_kit/js/toolbar.js shows a 404 when viewing page load in Developer Tools in browser.

Cause: symlink from vendor/cakephp/debug_kit/webroot to webroot/debug_kit is missing which causes the toolbar.js not to be found and loaded.

Resolution: 

[root@mypc cake3]# bin/cake plugin assets symlink

Welcome to CakePHP v3.2.5 Console
---------------------------------------------------------------
App : src
Path: /home/username/www/cake3/src/
PHP : 5.5.33
---------------------------------------------------------------

Welcome to CakePHP v3.2.5 Console
---------------------------------------------------------------
App : src
Path: /home/username/www/cake3/src/
PHP : 5.5.33
---------------------------------------------------------------

For plugin: DebugKit
---------------------------------------------------------------
Created symlink /home/username/www/cake3/webroot/debug_kit

Done