Exclamation point on plan guide
This is an educated guess, so if anyone has something more authoritative I am happy to rescind it.

I've never used the SSMS UI to manage plan guides before, so I immediately checked a few servers where I know I have viable plan guides setup:



The red Xs indicate the plans are disabled, as one might expect. 

When tracing SSMS as it populates this part of the UI, we can see it runs this query:

SELECT pg.name AS [Name]
, 'Server[@Name=' + quotename(CAST(serverproperty(N'Servername')AS sysname),'''') + ']' + '/Database[@Name=' + quotename(db_name(),'''') + ']' + '/PlanGuide[@Name=' + quotename(pg.name,'''') + ']' AS [Urn]
, pg.is_disabled AS [IsDisabled]
FROM sys.plan_guides AS pg 
ORDER BY [Name] ASC


Which only includes basic data and the disabled flag - nothing else. My hunch is that the yellow exclamations are actually the default icons for plan guides. 

This doesn't seem too far fetched since you're modifying execution plans and a similar icon is used whenever an execution plan might be impacted by something like a conversion or a plan guide: