(UPDATE 2) New post with a bit more information.
(UPDATE) NHK News says the mist generator was not in use this time.
It happened again. According to TEPCO's email alert for the press on August 19, 2013, an alarm sounded from the dust monitoring system in front of the Anti-Seismic Building at 10:04AM.
This time, two workers waiting for bus in front of the Anti-Seismic Building were later found with 13 Bq/cm2 contamination on their heads.
The dust monitoring result shows cesium-134 at 2.6×10^-4［Bq/cm3］ (0.00026 Bq/cm3), and cesium-137 at 5.8×10^-4［Bq/cm3］ (0.00058 Bq/cm3).
When it happened the first time on August 12, 2013, the water used in the mist generator was first suspected, but it turned out to be not the case.
There is no information whether the mist generator was in use this time or not.
The cause is still unknown.