So I just bottled an amber ale I had made - three weeks in primary, two in secondary (because I didn't yet have the requisite number of bottles stockpiled). When I took the top off of the carboy to begin bottling I noticed a slight sheen on the top of the beer. I have no idea what it was, it was something that was definitely not miscible, like an oil slick. It only covered about a third of the top of the liquid, I'm not worried about it, just curious. Any ideas?