With my first I was around 15 weeks I think, but I'm only 8 weeks now, and I have noticed my stomach has rounded back up already, probably because it's only been just on 6 months ago that there was a 3kg baby in there, more room to grow IFKWIM.
I don't think others can notice, however I do notice people who don't know yet looking at my tum, probably just think I'm getting fat.