试图回答一下，不保证对

I think you are right by saying "I found it".

becasue he or she, your colleague should say:

" I HAVE lost my pair of sunglasses"

why using the tense of perfect present? that means "NOW, i don't have my pair of sunglasses to wear."

现在完成时= 过去开始发生的动作，对现在的影响。强调的对现在的影响。

简单过去时，I lost my sunglasses, 只是表示过去曾发生的事情， 现在有可能已经找到了。或者我又买了一副新的。

and you should answer: I've found it

I cannot gurantee that my above answer is 100 percent correct.
I am not good at the grammar.
我是凭感觉说的。

请国内的同学回答吧， 他们是语法专家。