It's not trivial. As you can see from the statute I linked you to, if the offense can be proved against him beyond a reasonable doubt, he could theoretically be convicted of a class C felony.
There is a big difference between what is needed for the police to make an arrest or for the prosecutor to file charges and what is needed for a conviction. To arrest him /file charges, the state only needs something called probable cause. Probable cause does not require much evidence. It's just a reasonable belief that your brother may have intentionally caused damage to the elevator. Arguably, your brother's conduct could have provided the police with probable cause. On just that little the state is entitled to make an arrest if they think they may be able to prove their case.
To convict him, on the other hand, the prosecutor would need to believe not just that he caused the damage to the property but that he did so knowingly and maliciously. Not only that, they'd have to prove that beyond a reasonable doubt, which is a very heavy burden and which, based at least on the fact that you've told me, would be difficult for them to do. Again, however, when the state has probable cause, they are entitled to try.
So, one reason they kept your brother overnight was because the charge is not trivial. It's a felony with a maximum possible penalty of 5 years in prison. Another reason they kept him in overnight was that he was from out of state and would be presumed a likely flight risk.