The term "return line" is a bit confusing as there are no return lines per se, with the exception of the return water line on a closed loop instant hot water system.
On the other hand, i get the feeling that what you mean by "return" line is perhaps a short waste arm from the sink drain to the main branch line in your bathroom.
If the leak is coming from a drain line, they do have a miniature closed circuit video camera system where a camera and light attached to the end of a probe is inserted into the drain pipe and the operator can inspect the pipe from the inside out by watching the results on a monitor screen. Once the point of the leak can be located in that manner, the probe cable is carefully measured as it is pulled out of the line to pinpoint the actual point of the leak.
The same technique can be used to inspect inside a wall. A small hole is made just large enough to insert the probe, then they visually inspect the lines. Unfortunately that type of equipment is very expensive and usually is only available through the larger Plumbing companies, and it can be quite expensive to have it checked in that manner.
If the leak is coming from a water supply line, you can have some limited success by using a "Digital Laser Thermometer" to measure the actual surface temperature of the external wall surface. The digital laser shoots a red dot against the surface in question, and it digitally reads the temperature at the specific point of the dot. they are commonly being used in such applications as measuring the temperature of motor bearings while a motor is running, but can be used in this application. Normally if there is a pinhole in a water line, the inside of the wall board will be soaked and it causes that section of the wall to be hotter or colder than usual, depending upon whether it is a hot or cold water line leaking.
The alternative would be to physically open the wall for inspection. Often this can be done in an out of the way place where the opening will not be seen and can be easily repaired such as behind a vanity cabinet or through ad adjacent closet wall.