3.1 - Plugin generates 2 characters of unexpected output during activation

[jonathanwindle-zz]

Description

The following message appears on the WordPress Plugins menu when activating the plugin.

The plugin generated 2 characters of unexpected output during
activation. If you notice “headers already sent” messages, problems with
syndication feeds or other issues, try deactivating or removing this
plugin.

This is displayed in both single site and multisite installations

Steps to Reproduce

1. Install plugin in wp-content/plugins
2. Activate Plugin
   1. In single site go to the Dashboard then Plugins. Click "Activate" for the "WordPress CAS Client" plugin
   2. In multisite go to the Network Admin Dashboard then Plugins. Click "Network Activate" for the "WordPress CAS Client" plugin

Result

Unknown

Expected

WordPress displays the following message instead

Plugin activated.

@gissues:{"order":16.66666666666666,"status":"backlog"}