Caesar wrote in De Bello Gallico: "Ea nascuntur alces, animalia quae reliquis in locis visa non sint.". Why didn't he simply write "alibi" there? Is there a difference in meaning? And why doesn't "in" go before "reliquiis"? Did he maybe mean "by other (people) in (other) places"?
The difference is the same as the difference between "elsewhere" and "in other places" in English. Latin is a language, it has multiple ways to express a given concept.
Single-syllable prepositions, and especially cum, dē, ex, and in, will often be placed between the adjective and noun following it. You can see this as a kind of hyperbaton if you like, but it's idiomatic and very common.
Reliquis in locis is definitely a coherent prepositional phrase, and reliquis cannot mean "by others"—an ablative of personal agent requires a preposition ab/ā.