Irrigation backflow preventer drips only sometimes, normal relief or failure?

My residential irrigation backflow preventer has started discharging a small amount of water intermittently. It’s installed outdoors near the irrigation shutoff, and I’m trying to determine whether this is normal pressure relief or a sign that the device needs testing or replacement.

The discharge doesn’t appear to be continuous. I’m especially unsure whether it happens during or just after sprinkler cycles, or when temperatures change. I also haven’t determined whether sprinkler pressure has dropped, and there’s no obvious change in indoor water use.

For those familiar with cross-connection inspections:

- Can occasional discharge from a pressure vacuum breaker or reduced-pressure assembly be a normal operating event?
- What symptoms would point more strongly to debris, a failed check valve, excessive supply pressure, improper installation, or freeze damage?
- Should the irrigation system be shut off until a certified backflow tester checks it, particularly if the discharge becomes continuous or occurs when the sprinklers aren’t running?

I’d like to avoid replacing the assembly unnecessarily, but I also don’t want to treat a failed backflow preventer as harmless.