(UPDATE 7/23/2013) TEPCO measured the radiation levels around the area. See my latest post.
It rained in the morning at the plant. So, just like the previous incident on July 18, this one is explained as rainwater hitting the warm (about 40 degrees Celsius) Containment Vessel Head.
From NHK (7/23/2013; part):
"Steam" rising again from Reactor 3
At Fukushima I Nuclear Power Plant, what looks like steam is seen rising from the top floor of Reactor 3 building on July 23, following the same incident on July 18. As there is no change in radiation level above the building, TEPCO thinks it is the rain in the morning that seeped inside the building and warmed up. The company continues to monitor the situation.
According to TEPCO, the image captured by a video camera installed [on the top floor of Reactor 3] showed what looked like steam rising from the floor of the Reactor 3 building on 9AM on July 23.
There is no change in radiation level above the building, and the cooling of the reactor and the Spent Fuel Pool is stable. There is no data that indicate abnormality, according to TEPCO.
Since the steam was first observed on July 18, TEPCO has stopped the work to clear the debris on the operating floor of Reactor 3.
Nuclear Regulatory Authority told TEPCO to take infrared thermography of the location under the condition similar to the fist incident (raining), but TEPCO only heard the first half of the order - take infrared thermography. And take they did, on a sunny day on July 20.
I wonder if they had wits about them to take thermograph today (July 23), when it was raining. It looks like TEPCO will have another chance on July 24 and 25 (weather forecast in Fukushima is rain).