(I didn't even know that SARRY had a vent line.)
The cause of the leak seems to be elementary (yet again). They had a plug in the drain line, so the contaminated water that should have gone to the drain line went up and out of the vent line instead.
What has happened to "attention to detail" which has been a hallmark of Japanese processes? (Or "was" a hallmark?)
Fukushima I Nuke Plant is entering the second winter since the nuclear accident, and many, many more winters to come until the plant is decommissioned. Well beyond the attention span of most Japanese.
From TEPCO (11/20/2012):