Thanks to +Ana Hoffman by way of +Mary Ann Davis then +Luiz Fernando then +John Elstone (love the RIPPLES on Ana's original post!) for sharing this tip.
To find private messages in Google plus, simply:
1. Do a regular Google search in your Omnibox, either for a person or a keyword or a combination.
2. Under Search Tools, select Private.
Here's Ana's post. Feel free to leave positive comments for her!