It's important to remember that Mutual Aid agreements are NOT signed by the providers, but by the local government.
In other words, if city "A" and city "B" sign mutual aid agreements with each other, that agreement applies regardless of who provides the service to cities A and B.
So, it's not a matter of whether or not RM has an agreement (their only agreement is with the city), but whether Aurora has agreements with anyone else.
One more thing, the RM units that are seen in other cities may be RM units that are NOT part of the Aurora system/contract. In other words, just because a RM unit is seen taking a call in Denver, that doesn't necessarily have anything to do with the city of Aurora.