If an email or note is attached to a timeline of a public contact and/or their work, all of your colleagues will be able to see it.
In Triage, you can always check which of your colleagues have access to see the conversation by clicking on the colleague's tab in the information tray.
If you'd like to keep the conversation private, just deselect the timelines by clicking on the 'X' under the Timelines portion of the Information Tray. The email will no longer show up on the timeline and is now only visible to you.
Keeping emails private
How can I tell if an email is visible to everyone or just me?
How do I find out who has access to an email conversation?
How do I keep an email from a particular contact private, if I have the auto-share email setting turned on?