The detector wouldn’t be in a drawer tray. Most likely inside where the laser is. Probably an optodetector peeks through a hole in the tray and if disc is present, would reflect back. Or perhaps the laser itself sends a detection pulse to see if a disc can be detected. Whatever method, you probably should refer to the service manual if you are serious about trying to fix it. The information about what system is used for disc detection is probably in there. Unfortunately, parts might be the biggest obstacle to keeping these running.