There is a planet that is full of underground caves. Some are narrow and long, others, go on for miles and are wide enough to contain entire cities. However, all of these caves, are completely dark.
One organism, commonly grows to cover just about every surface. It nourishes itself off the heat of the planet and the water which has been endlessly falling through the tunnels, and acts like a cooling system for the interior caves, whereas the surface is a scorched, barren wasteland that is unable to support any type of life. This organism tends to spread any excess water it encounters to its neighbors, resulting in cavern surfaces that are essentially covered in a film of material that helps reflect any sounds from the various lifeforms that inhabit the caves.
One particular species, called species "A" is not all that different from us humans. They started as simple caretakers of a certain plant. This plant could easily grow in the conditions of the caves, but they tended to consume large areas of space. One plant could grow to encompass the area of a football field, and the pollen it produced would be packed and stored by the A into structures not unlike a bee's nest. However, the plants generally killed the organisms mentioned above, and required the water that they spread, so the A needed to depend on multiple plants, carrying and moving the offspring to new locations after successful harvest (and accidental pollination). The long time the plant took to grow meant that the A spent a lot of time in a migratory pattern - starting new plants and then returning across vast distances to harvest the older.
Unfortunately, the A also tended to compete among themselves, and also had predators to deal with, along with other animals either trying to eat the plants, or break into their homes. They could escape many larger predators by hearing them and escaping into smaller tunnels, but smaller threats required some thinking and experimenting with a variety of tools. In the end, certain chemicals were found to be highly harmful to most underground creatures, along with certain rocks which, after being rubbed together, could create an incredible heat that ate some materials(fire) - even away from caverns devoid of any cooling organism.
From here, they managed to continue making processes and ideas simpler and more effective. They learned how to construct better and plan more effectively, as well as develop defenses for any opposing A's that may try to steal their food.
While developing into what would be humans' Renaissance Era they discovered a new environment. The temperatures here were not as severe, making our previous organism unable to grow. Instead, there were tall, centrally located plant spires which stored water and gave off a strong bio-luminescence, enabling many creatures in this area to develop eyesight, which is a rare feature on this planet.
In this area, sounds could be heard, even with their reduced hearing (from lack of the cooling organism spreading across everything) and they knew things were out there - yet they seemed unable to catch any of these strange creatures, or even get close to them. They were considered ghosts, or spirits, and A's rarely went into the area, especially because of how much their hearing ability was reduced - making them feel as if they were just bumbling about in the dark ;)
I'd like for the humanoids to somehow discover that the other animals are using sight in the manner that our humanoids use sound in their own caves.
What is the earliest and most likely way (starting from the technology level of the Renaissance Era) that our humanoids would be able to discover that the animals are using a different method of detection (sight), and then successfully accomplish catching one of these animals?