Chatter enforces security and sharing rules in Salesforce. Let’s say you make a post to a record or a private or unlisted group and @mention someone who doesn't have access to that record or isn't a member of the group. In this case, the @mention appears as a gray link. The mentioned person can’t see your post and doesn’t get notified about the post.
For example, Madison creates a private group for her project team and forgets to add her coworker Sandy Dunn to the group. When Madison posts an update to this group and @mentions Sandy Dunn, Sandy isn’t notified about the update. Sandy also and can’t see the update because she's not a member of the private group. If Madison later adds Sandy to the group, Chatter doesn't notify Sandy about the previous mentions.
There’s an exception: if you @mention yourself, you don’t receive a notification.