Ttykuu

In my world there is a village with a hot spring, the only one on the entire island.

The spring emerges from a local cave which has been filled / caved in by a past civilisation, since the water is secretly heated by engineered means.

The spring has been hot since neolithic times, but thousands of years later the inhabiting nation becomes gripped with a frenzy of curiosity and exploration. They have all the means of current technology apart from non-intrusive imaging (such as sonar or X-ray).

What could prevent them from exploring the source of the spring, ideally without raising much attention?

Risk of cave-ins

As you stated in your question, the cave which contains the technology has already caved in at least once in the past. The local populace may be fearful of the cave being unstable and, if they were to disturb it by delving inside, the cave could collapse and kill the explorers. This would discourage them from attempting to explore, the reward of knowing whats inside there is not worth the risk.

Risk of Volcanic Activity or Extreme Heat

In a similar vein, the populace may be fearful of exploring the cave as they may be walking into an active volcano. Obviously we know that it is ancient technology that is heating up the water but they wouldn’t, likely assuming the heat to be caused by underground lava lakes or intense geothermal activity.

There may actually be lava lakes or spots geothermal activity down there that the ancient technology is utilising for power (maybe the hot springs are actually a way to vent off the heat produced to prevent damage to the machinery). If someone went into the cave and felt an extreme heat or saw a lake of lava, they may think twice about exploring it further.

Flooded Cave

You mentioned that the cave may instead be filled in. If it was filled in with water, that could massively deter people from exploring it. The reason being is that not only is it dark, it is extremely easy to get lost (as now you are moving in a 3D plane, rather than just along the cave floor), you have a limited supply of oxygen, plus you can only go down so deep before the pressure on your body from the water around you is too great.

Your ancient tech may be safe and sound in an air pocket further in but divers would never be able to reach it. They wouldn’t know the way there and, even if they did, the way there may be too dangerous and it prevents them from reaching it. Whilst a submarine might be able to do it, the caves are too small for one to fit in.

Religious Significance

The hot spring may be a holy site, the cave being sacred. To enter the cave is to desecrate the holy site and is punishable by death. This would prevent people from attempting to explore the cave, perhaps out of respect for the religion or perhaps out of fear.

A modern day equivalent of this is a church in Aksum, Ethiopia claims to house the Ark of the Covenant. No one is allowed to visit the site, the monk who watches over the Ark must do so until he dies, he is not allowed to leave the chapel grounds. Your hot spring and the cave may have a similar religious significance, preventing people from exploring it.

Toxic Gas

There are some heavier-than-air gasses which could cause the cave to be inhospitable. Carbon dioxide is one such gas, if the lower parts of the cave were filled with carbon dioxide (or worse, carbon monoxide) then people would start to choke and feel dizzy as they went further and further, eventually dying due to asphyxiation.

The phrase ‘canary in the coal mine’ comes from this problem. Canaries are small birds that were carried into mines by miners. The reason for this is because the small birds quickly panic if they breathe in toxic gasses. Also, as the birds have much smaller lungs than humans, it takes less toxic gas to kill them. If the canary died, the miners knew they needed to get out of the mine quickly before they did too.

User Liam Morris already gave a very comprehensive answer, yet there is one possibility that I would like to add.

There is no reason to investigate the spring

The hot spring just looks like a regular geothermal spring unless one is willing to put extraordinary effort into exploring it. Make the island have a volcanic past, like Island, Hawaii or the Galapagos Islands. The machines creating the heat are very deep down, maybe 5 km or so. A meters thick diamond skin should serve to protect the equipment at that depth and it and the rock provide radiation shielding, preventing anyone from catching the signatures of the presumably nuclear energy source. (I assume nuclear, because how else would one power such a thing? Could shield magic, too.)

The construct heats water which then travels through the rock layers, making it look like ordinary hot spring water.

After all, there is nothing special about it, there is no reason to investigate.

PS: This TED Talk about a boiling river in the Amazone might be related/interesting. https://www.youtube.com/watch?v=k4N2SxUZwiU