1. Using "it" (most common):
* They noticed it. (This assumes the context is clear that "it" refers to the light).
2. Using "them" (if referring to multiple lights):
* They noticed them. (This assumes the context is clear that "them" refers to multiple lights).
3. Using a specific object pronoun if there is more information about the light:
* They noticed its flickering. (This focuses on the quality of the light)
* They noticed its color. (This focuses on the specific characteristic of the light)
The best way to rewrite the sentence depends on the context and what you want to emphasize.