A wizard casts Locate Creature looking for any human, if in that moment in range there isn't any human nothing should happen. When a human enters in the range and there is not lead in his direction, the caster dicovers the direction of his location.
Does the spell continue pointing to that human when other humans arrive nearer to the caster than the first one?
What if the first human in range goes out of the range of the spell and another one enters the range?
My interpretation of this spell is that it would fail instantly if you were unable to locate a creature as it is cast. Therefore if humans show up (within 1000 ft) after that, you are not currently concentrating on the spell since it already failed, so you wouldn't know at all (unless you cast it again).
The spell description (emphasis mine) says:
The spell can locate a specific creature known to you, or the nearest creature of a specific kind (such as a human...
If you are able to locate a creature the instant that the spell was cast, the concentration is to keep tracking that creature, so others that may be closer to you in the meantime over that hour are irrelevant, since they are not the creature that was identified as the spell was cast.
The spell description (emphasis mine) also says:
Describe or name a creature that is familiar to you. You sense the direction to the creature's location, as long as that creature is within 1,000 feet of you. If the creature is moving, you know the direction of its movement.
From this, it looks as though the human located (assuming they were within 1000 ft when the spell was cast) would cease to be "tracked" if they move further away than 1000 ft of you, so at that point I believe the spell should end (and thus you are no longer concentrating on it) so other humans moving within 1000 ft of you wouldn't be tracked because the spell has ended.
This spell is ambiguously worded.
[...] You sense the direction to the creature's location, as long as that creature is within 1,000 feet of you. [...]
This first sentence can have at least two meanings. One possibility is that the spell fails once the creature you are tracking leaves 1000 feet. Another is that you only gain the benefit of knowing where the creature you are tracking is when they are within 1000 feet of you.
Both interpretations are consistent with the sentence. Call these interpretations (A1) and (A2).
If the creature is moving, you know the direction of its movement.
This separate sentence could be read as connected to the previous qualifying clause (as long as the creature is within 1000 feet of you) or not. Both are consistent with what this sentence could mean. Call these interpretations (B1) and (B2).
In the first interpretation B1, you don't get any information when they are 1000 feet or more away. If we interpret both in the second way (A2 and B2), we get location information within 1000 feet and direction of movement information with unlimited range.
Note that B2 and A1 don't make sense together.
The next question is, can the creature change? The first sentence of the first two paragraphs of the spell seem to talk about this:
Describe or name a creature that is familiar to you. [...]
The spell can locate a specific creature known to you, or the nearest creature of a specific kind (such as a human or a unicorn), so long as you have seen such a creature up close--within 30 feet--at least once. [...]
There are again two possible readings here. The first is that when the spell is cast, a specific creature is fixed. This creature is either a specific one or the closest creature of the described type. Call this interpretation (C1).
The other reading is that when you describe a creature, you always and continuously are tracking the "nearest creature of a specific kind". When the nearest creature changes, so does the creature you are tracking. Call this interpretation (C2).
Both (C1) and (C2) are consistent with all of (A1) (A2) (B1) and (B2) (excluding of course (A1, B2)). Both (C1) and (C2) can be useful in different situations, but I suspect (C2) will be more useful (as it can be used to grant you a more reliable "danger-sense" radar).
However, there is no reading of the spell that has the human being "locked on" the moment it enters 1000 foot radius.
Let's walk through the scenario:
You cast the spell saying "nearest human" with no humans within 1000 feet.
A1: Spell fails. From this you know there are no humans within 1000 feet.
A2 B1: Spell finds nearest human, you are told no humans within 1000 feet.
A2 B2: Spell finds nearest human, you know which way that human is moving but not where they are
You maintain concentration if the spell didn't fail. Then, that specific nearest human moves within 1000 feet.
Another human moves closer to you, say in the opposite direction.
A2 C1: You are told nothing about this other human.
A2 C2: You are now told the closest human is in another direction, and which direction this new human is going.
All humans leave 1000 feet, and a different human approaches within 1000 feet.
A2 B1 C1: You don't know any humans are within 1000 feet, but you know your human is further than 1000 feet.
A2 B2 C1: You don't know any humans are within 1000 feet, but you know your human is further than 1000 feet and which direction they are going.
A2 C2: You are told about the different human's direction and movement direction, and know nothing about the original human (even that you are now tracking a different one).
Personally, as a DM, I'd go with A2 B1 C2 interpretations.
If there are no targets within 1000', it gives no information about them, but the spell doesn't fizzle or anything. When you pick "closest human", it always tells you about the human that is closest, not the closest one at the point you cast it. If you pick "Bob", it tells you only about Bob.
But the real answer is "ask your DM".
If you cast the spell, and there is no one of the type within the 1000' radius area of effect (not range), you detect nothing, but that does not mean the spell fails and ends immediately. It's a Concentration spell, with a range of "self", so the caster should be able to maintain concentration for the duration if he so chooses, and detect creatures that enter the area of effect.
Assuming that it was not cast to detect a particular creature, but rather as in your example, the closest human, then what happens is a DM ruling. I would say that it would always track the closest one at any given time, so if one enters, it's him, and one enters and comes closer, then it switches to that one, etc.
