When you are using a custom status, Plus! actually adds that to your display name. If you go away, and choose Fishing as your custom status, Plus! will change your status to Away and make your display name "Old display name {Fishing}". This is what you see: that added to your name.
To people who have Plus!, the {Fishing} is shown as (Fishing) instead of Away, because their Plus! recognises it as being a custom status. Someone who does not have Plus! installed will see "Old display name {Fishing} (Away)".
If you would add yourself to your contact list you would see "Old display name (Fishing)" in your list as well, because Plus! recognises it