IPhone 5 already have stored hidden files?

IPhone 5 already have stored hidden files?

Added (1). So i just got a 'new' iPhone 5c from work, i know its not brand new because it has been updated to 8.3, so maybe its second hand but it is in good condition and other than the 8.3 update it is in factory settings. Although i looked at the size of the phone noticed it was 5.4gb, which is typical as the system files take up about 2gb but then it says it has 400mb of storage used, however there are no apps, no musics, pictures etc, so what could be using the storage? Hidden files? To tap calls?

Well since it doesn't run on dirt I would guess it needs files.