The Sigil behavior is to delete unmanifested files because they are invalid according to the standard and normally unusable.
In the vast majority of cases, it doesn't matter because they are usually, like here, useless artifacts left by other software... but sometimes not. Sometimes, the ePub was badly constructed and some of the unmanifested files ae important.
This window was added to warn you and check that you won't delete anything important in this ePub by continuing to use Sigil. It is better to be overly cautious rather than silently deleting this files, as Sigil's old behavior did, which could be disastrous.
Otherwise, for this iTunesMetadata.plist, as the others said, if you want to see it, you had to rename the .epub extension of your file to .zip and open it as any compressed folder... but it's a lot of things for not much.
iTunesMetadata.plist contains redundant metadata with the OPF (and again, just the basic one), nothing useful, you could abandon them without fear.
Here an example I found: