According to TEPCO's press release on September 17, there were increases in radioactive cesium in the sub-drain of Reactor 1 in late July, then mid August, and in early September. From mid August to early September, iodine-131 was also detected.
This happened most noticeably in Reactor1's sub-drain, but small increases are discernible in Reactor 2's sub-drain, too. (For the sub-drains for other Reactors, see TEPCO's PDF.)
By the way, in a separate presentation, TEPCO says it doesn't quite know exactly what the pressure or the water level is inside the Containment Vessel or the Reactor Pressure Vessel. In the press conference, they didn't quite seem to know what temperature they were measuring. The presentation is in Japanese only. I'll link if they post the English version.
TEPCO and the government are talking about what to do when the "cold shutdown" is finally achieved. How they determine the "cold shutdown" without knowing the pressure or the temperature remains mystery. They are in no hurry to find out where the corium may be, either.