started hive from package, fed bees sometimes daily thru summer. Stong hive with 1 brrod chamber, two supers full of honey. Took 1 super off two weeks ago prepareing for minimum space for winter. Bee keeping class graduate last winter so have some better than average knowledge of beekeeping. inspected every two weeks all summer and fall. Friday, noticed a robbing attempt going on by what appeared to be smaller feral bees. front entrance closed down to 3/4 in to allow only small area of attack so my bees could defend for the last 2 weeks. today i went to check on hive and found maybe 50 dead bees on bottom screenboard, 30 or so at entrance, 15 or so on top board, and entire hive empty of bees. empty cluster space in brrod frames with capped honey and pollen stores around top and sides. Top supers are still capped honey. no ragged edge uncapped honey stores. lots of stored pollen. a few yellow jackets were crawling in and out of hive entrance and scattered throughout. inspection 2 weeks ago saw nothing out of ordinary. no varroa mite problems, a very few hive beetles. no evidence of disease. 2nd hive 4 ft away still ok...no signs of problems. Anyone have an opinion what happened? or even anything similar? Central alabama here, 1st cold snap started yesterday....average fall. hives well protected with 8ft wind break.