I consider those markers to be ”ToDo" placeholders … indicating datapoints that need to be manually compared again against the original data source.
(It is worth running an Event custom filter for events with no references. There’s a possibility that the event had been created but Gramps was interrupted before linking it. That can happen with an abnormal exit from the application; or, if you close the parent {Person or Family} dialog before the spawned child {Event} dialog. Gramps WARNS when that happens and offers to save the spawned data… but forgets to actually link that saved data.)
But the missing media objects might have simply been moved in the OS, outside of Gramps. There are tools specifically for Gramps that locate moved files and fix the link.