No featured image set but thumbnail ID still returned

On an existing post. I have no featured image set, but yet my has_post_thumbnail statement is always returning true, outputting image markup which is not needed. This is happening on the post single template.

A dump of get_post_meta( get_the_ID() ) returns a thumbnail_id:

'_thumbnail_id' => 
    array (size=1)
      0 => string '6721' (length=4)

But yet there isn’t even an image in the media library with this ID. I only have 5 images uploaded (attached elsewhere), and not to this post.

Think I have narrowed down the issue for those interested.
All posts on this site were imported using the Wordpress Export/Import tool. Subscribers too.

Subscribers do not have permission to add media on this site, so whilst the post was imported, media was not but the post does not know that, so its meta_data still looks for the original media ID’s.

Or maybe we were importing from an old WordPress to a newer version and something failed with the import. Was done months ago so no records to check unfortunately.

Current Issue
I now need to find a way to clear the meta_data between the posts and the old media ID’s. Or add a further conditional to check for blank URL’s and return false.

For now, I simply uploaded a new Featured Image to the post. Saved it. Then removed it and saved again. This then updated the database to say, this post does not have a featured image, therefore returning False to the original has_post_thumbnail statement.

I am done with this now, feel free to post updates on other approaches in future though.