Thanks for noticing. You're sharp.
If a mate or a friend pays attention to something or becomes aware of something very small, you would be glad to.
You can say in Korean as below.
눈썰미 좋다.[nun-ssŏl-mi jo-ta]
눈썰미 [nun-ssŏl-mi] 좋다 [jo-ta]: observant; have an eye for details, have a fine eye
