When I am away from Trello (out of office, at lunch, etc), only way I can tell if I have been mentioned in a card I have not viewed recently is to scroll through past activity in Outlook. Is there a switch in Trello, or a plugin that would highlight a card that I have been mentioned (tagged) in, or a card I created that has had activity? Basically trying to figure out how to make sure I do not miss activity in a card that has mentioned me. Seems like a fundamental feature that probably already exists but I am not sure if I have settings correct.
By default, notifications should appear in the notification center as well as a little red notification bell on the card where you were mentioned or that you were subscribed to.
If you open up the notification center, it clears out all of the notifications. So if you want to process them one by one, you might want to change your notification settings to "Instantly" (Click your avatar and then click on "Settings") and use your inbox in Outlook to address them.
Perhaps I'm misunderstanding what you need. There is also an option to subscribe to a card, list, or board. If you do any of these three, you should receive all activity updates corresponding to what you subscribed to. (i.e. only card updates, list updates, or entire board activity)
In my experience, Trello's notifications are useless because they have no granularity. It makes it too hard to notice the notifications I actually care about.
A workaround I created to address exactly your question is to use Zapier and Slack. I created a private channel in Slack called my-trello-mentions. Then I set up a zap that is triggered on New Notification (Trello), a filter (only continue if Data Text contains @[myusername]), and an action Send Private Channel Message that contains the parts of the message I want (board, card name, member creator full name, data text). So now I have a channel that only includes comments I'm mentioned in, and I set Slack's notifications (which are very granular, like Trello's should be) so I get notified of posts in that channel.