A plant that is too dry will recover within a few hours when watered. Sometimes a cold draft will cause wilting too. Overwatering can also cause a plant to droop. An overall yellowing of a leaf or a few leaves usually indicates overwatering. Occasionally, yellow leaves can develop when a plant is too cold.
Chinese evergreens are sensitive to fumes, especially from oil or gas.
The patches will be irregular and both along the edges and within the leaf. Sometimes fertilizer that is too strong will cause brown edges. First check to make sure the plant isn't severely pot bound. If it is, repot it. The other likely cause is lack of fertilizer.