So I have two cards that say "Return two target creatures to their owner's hands," but there is no target location from where those cards may be. So could I recover two creatures from exile, and put them into my hand? What about my graveyard?
A "creature" by definition means a permanent on the battlefield.
109.2 If a spell or ability uses a description of an object that includes a card type or subtype, but doesn’t include the word “card,” “spell,” “source,” or “scheme,” it means a permanent of that card type or subtype on the battlefield.
So whenever something says "Target creature", it always is referring to the battlefield.
Whenever a spell wants to refer to a card in another zone, such as the graveyard, it will use the phrase "creature card" or "creature spell". See Reanimate as an example.
Worth noting that there is another, somewhat redundant rule, dealing with targeting specifically:
114.2 Only permanents are legal targets for spells and abilities, unless a spell or ability (a) specifies that it can target an object in another zone or a player, (b) targets an object that can't exist on the battlefield, such as a spell or ability, or (c) targets a zone.
This rule clarifies that if the spell isn't specific about targeting something in another zone, etc, then it can only target permanents.
110.1. A permanent is a card or token on the battlefield.