When you where away, you are not breathing in your fish room - creating CO2, which is absorbed into your tank, thus dropping the pH. (Seriously, I have seen this many times at old reefer meetings at peoples homes when 10-12 people standing around a tank for an hour or two, the tank pH drops due to (often heavy) respiration)
I would be breathing heavy too... If I was staring at your corals (drooler)