First of all, make sure you know that it is MotoTrbo. There are several digital modes that will sound similar.
Second, there currently are no consumer scanners (portable, mobile or base) that will decode MotoTrbo, NXDN, etc.
Access keys are usually only needed for trunked systems, and there are needed to transmit into the system. When there is a consumer scanner available, you won't need it to receive.
There are programs that can run on a computer, take an discriminator tap off the radio, and decode the digital signals, if it isn't encrypted. The other option would be to purchase a transceiver capable of working with whatever digital mode it is, and programming it correctly. The transceivers are not cheap, require programming software and cables, and will not necessarily guarantee reception. You would have to know -exactly- what mode it is before making a purchase.