In the first sentence, the fire predates the discovery. So in this sentence, the fire is no longer burning. In the second sentence, the fire and the discovery occur in the same time period, so the fire is still burning.

Personally I think they both mean the same thing.

第一个是过去完成式，已经结束，对现在木有影响。第二个是过去式，而且很有可能是昨天才发生的，或者还木有被熄灭，大概就是这样le·~

Example 1: i think the first sentence is certain or sure that there is a small fire. the 2nd sentence is not sure, they just alleged that there is small fire. Example: the sentences is in the past but the first sentence refers that the fire is starts in the past but the room is still burning. the second sentence is refers to allegation,that there was fire .