No, the hints were indeed wrong. Fixed now (may take some time to implement).
It is a good habit to double-check the declension on Wiktionary, especially that the hints do not necessarily apply to the sentence you see at the moment, but to the word itself. Or sometimes there's just a mistake.
Despite its name, Locative is not only used for denoting location (one of its main usages is after "o" meaning "about"), and it's also not the only case used for location, actually. A portion of quite basic prepositions describing location takes Instrumental. "za" is one of those.
Does that mean that the preposition "za" should be always be followed by a noun in the "instrumental form" ? While in the "Locative case" not all prepositions should be followed by the locative case ending even though it is indicating location or "about". Like "Do" and "Po"? It is a bit confusing, sometimes after the "Do" the noun takes the Genitive case while on other sentence structures it uses the Locative case. Thank you for your reply and hoping for any further answers.
A preposition in general can take different cases depending on its exact meaning in the given context.
The basic meaning of "za" is "behind" (Something is behind something), and that takes Locative.
Then you can have movement 'to behind', so to say. For example "I am going behind the house" (I was at the front and I am moving to the area behind the house). This takes Accusative.
Polish Wiktionary mentions 8 other usages of "za" ;) Just like in English those 'small words' can have multiple meanings.
"do", on the other hand, unless I'm forgetting something, takes Genitive in any context.