If your beer was low in diacetyl when finished fermenting, you can get it from:
1. An infection, but you said you are on the sanitation thing.
2. Exposure to air at packaging. O2 will oxidize the diacetyl precursor into diacetyl. When I read that in "Yeast", I realized what happened to my Second Round Pilsner last year. The keg was fine, and it scored high in the first round. I got rushed bottling and did not purge the bottles with CO2, maybe was a little rough getting them filled. I had bottled a few extra, and the one I drank after I got the scoresheets had enough diacetyl that I could pick it up, even with my medium high sensitivity to it. Live and learn.