Here is one more thing you can do. I installed a 12VDC counter in line with the bildge pump/float switch. This way, when I come back to the boat after a week away, i can see the exact amount of times it came on. I reset it when I leave for a long period and I write down my findings in the Log whenever it's abnormal. So far so good. It cost me about $14.00 at H.R. DISTRIBUTORS, Inc.
Also, the time you pumped the boat out, it could have been fresh water.... keep your eye on the mast if you have a keel stepped mast and an eye on all the port holes..