Query Plan showing missing index warning on trivial statements
Without seeing the plan and based on the information you provided, it sounds like you don't have that column, OrganisationID, in one of your existing indexes.  Foreign keys aren't automatically indexed, so if it is part of your join or in the where clause then there could possibly be a benefit of adding the index.  The missing index feature isn't always right, so you should always review whether you really need the columns suggested indexed or if you can modify another index to include the columns instead of adding a separate index.

You can use this query from Glenn Berry to look at the missing index dmvs and see what the index advantage, unique_compiles, and user_seeks to help make  the decision:

select user_seeks * avg_total_user_cost * (avg_user_impact * 0.01) as [index_advantage]
    ,migs.last_user_seek
    ,mid.[statement] as [Database.Schema.Table]
    ,mid.equality_columns
    ,mid.inequality_columns
    ,mid.included_columns
    ,migs.unique_compiles
    ,migs.user_seeks
    ,migs.avg_total_user_cost
    ,migs.avg_user_impact
from sys.dm_db_missing_index_group_stats as migs with (nolock)
inner join sys.dm_db_missing_index_groups as mig with (nolock) on migs.group_handle = mig.index_group_handle
inner join sys.dm_db_missing_index_details as mid with (nolock) on mig.index_handle = mid.index_handle
--where mid.[statement] = '[yourdb].[dbo].[OrganisationLocations]'

order by index_advantage desc
option (recompile);