But actually, scratch that. We're in the modern era now. Send a drone into the outer pipe, and use some sensors (perhaps also video) to detect excessive moisture, or corrosion, or whatever. So it's no longer expensive.
Most new pipelines are inspected currently with "drones", called smart pipeline inspection gauges, or smart PIGs, that use ultrasonic or magnetic flux to measure the wall thickness over the entire length of the pipe [1]. The PIGs are propelled through the pipe via the working fluid in the pipeline.
I think creating a similar device to travel in the annulus of cased pipe would be much more difficult. The centralizers used to keep the inner pipe centered in the outer pipe would pose a pretty big obstacle. Plus, there would be nothing propelling it forward, and communicating with the device through a steel pipe would be very difficult.
[1] https://primis.phmsa.dot.gov/comm/FactSheets/FSSmartPig.htm?... [2] https://kwikzip.com/ (example)