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."
現在完成時= 過去開始發生的動作，對現在的影響。強調的對現在的影響。
簡單過去時，I lost my sunglasses, 隻是表示過去曾發生的事情， 現在有可能已經找到了。或者我又買了一副新的。
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.
我是憑感覺說的。
請國內的同學回答吧， 他們是語法專家。
