Is it possible for items one hasn't ever downloaded to appear in the download history of an s5 mini?
Yes and no. Android's Download Manager gives apps a way to ask it to download files for them. Those downloads still show up in the list of downloads within the Downloads app. Sometimes they might show a notification while they're downloading or when the download finishes, but that's up to each particular app.
So all the items that show in the Downloads app are files that the Download Manager has downloaded, but not all of them are files you have asked it to download: some items can be added to the list and downloaded without your input.