I would prefer the use of an external scanner/radio to an app, if I were to use one at all, for two reasons:
1. I'm not going to have my phone pinging cell towers while I'm out exploring, and actively tracking and logging my location. I'll give someone I trust my location in case I don't come back, or go out in pairs for safety, but I'm leaving my phone elsewhere or at least switching to airplane mode.
2. Most (but not all) modern police chatter is digital and encrypted specifically to prevent exactly this. This means that a cheap analog FM-only scanner from Amazon will likely not pick up what they're saying. If you're lucky enough to be able to receive whatever digital protocol they use, you still wouldn't be able to decrypt it. At best, you would pick up noise when they are talking but have no idea what they are saying. You
may get lucky in a rural town or county with a poorly funded constabulary still using unencrypted FM, but the chances of that are slim next to none. Contact your local ham radio club, or search the area on
Radio Reference. They'll know.
Sorry to resurrect an old thread yet again, but this seems to be a semi-common occurrence so I'm assuming it isn't against rules.