The local gang was absolutely unscrupulous as far as Lu Bei could tell, especially its leader Zhao Xiayang who has a mean streak. The moment Zhao led his dozens of disciples into the ancient ruins, Lu Bei had already blacklisted Qing Shui Gate.
After entering the ruins, Lu Bei encountered several bizarre adventures, pursued by various mechanisms. This made him suspect that Qing Shui Gate wasn't as simple as they appeared. It was highly likely that they had already explored the ruins and located the core area that controlled the mechanisms and traps.
If not, Qing Shui Gate definitely had a strong connection with the ruins. The mining operations apparently conducted by another sect were actually closely linked with Qing Shui Gate, making them all local tomb guardians.
Moreover, if he guessed correctly, General Wang had also suspected something.