Planting evidence

John commits argument from personal incredulity (There is no way that they will attempt to hide weapons).

in addition, saying that the police force is known to plant evidence is a non sequitor, even if it were true that the police have previously planted weapons,  it does not follow that they planted weapons in this case.

I think Ken is guilty of irrelevant conclusion (ignoratio Elenchi). A judges warrant does not mean the students are guilty of hiding weapons.

I agree that John is making an argument from personal incredulity. However, there could be merit into what John says if he has some statistics about the local police planting evidence. He can strengthen his argument if he offers reasons why the jury should doubt the credibility of the warrant even if the judge issued it. 

If this were the case, Ken might be making an argument from personal incredulity (judges know better; judges don't lie; the experience of a judge overrides statistical evidence). Moreover, Ken would be committing a circumstantial fallacy because the fact that activist hide in a "convenient" place doesn't show that they actually hid the weapon (it all depends on how much evidence John presents).

The more we know about the case, the better we can tell who is committing a fallacy. 

John is offering two lines of evidence to support his conclusion.

1): There is no way that [the students] will attempt to hide weapons in a place surrounded by many houses.
2): The police force is known to plant evidence.
C: Therefore, the evidence against the students was planted.

The conclusion is not supported by the evidence presented. If 1 is true and there and the students did not attempt to hide the weapons, it doesn't mean that the police did plant the weapons. There are other possibilities (this could be an implied False Dilemma ). If 2 is true, this isn't strong enough to justify the conclusion that they did plant the evidence in this case. This appears to be an Appeal to Possibility given that they planting evidence is far less common that actually finding legitimate evidence.

Ken is also offering two lines of evidence to support his conclusion:

1) A judge issued a search warrant hence there is probable cause that those students were indeed hiding weapons. 
2) Precisely that mindset that their location is surrounded by many houses that nobody would suspect that they are hiding any weapon. 
C) The evidence was not planted.

1 is good evidence against the conclusion that evidence was planted, but that it was certainly not planted. 2 is an example of Ad Hoc Rescue or even a Self-Sealing Argument .

In both cases, John and Ken are speaking like lawyers and drawing unreasonable conclusions based on the presented evidence. To be more reasonable, they should use "most likely" in their conclusions.