To my delight,I found the new flat -------- in pleasant surr

你分析得没错,一般当 XX 作为主语后接 be located in ...的句型的确是固定用法.
但是,这句里因为 I found 的关系,the new flat 成为句子的宾语,be 就必须略去啦.
比较：
The gift was hidden in the closet.
I found the gift hidden in the closet.
I found that the gift had been hidden in the closet.