In Develop Module under the "Detail" section, why do I have an Exclamation Mark ( ! ) in upper left corner of "Detail" Section? It goes away when I enlarge image but comes back when shrink down size to normal size, WHY? I just noticed this. Can any one help me with this situation?
LR is telling you that sharpening and noise reduction settings should be adjusted at 100% view to get the best results, not at a reduced view. You can get rid of the exclamation mark by viewing the image at 100% or by opening the small 100% image on the right panel itself, by clicking the small l triangle/arrow on the upper right corner of the detail panel.
This behavior is well explained on LR documentation, which, BTW, makes for a nice reading if you are not familiar with LR.