Xenon was detected in the order of 10^-5 (ten to the power of minus 5) per cubic centimeter, says TEPCO's Matsumoto in the press conference that is on-going right now in Japan. It is significant enough to conclude it is not from March. (TEPCO by the way started to netcast its press conference live, here.)
Matsumoto does think a localized, small-scale, and/or temporary nuclear fission may have occurred in Reactor 2. But not a large, sustained re-criticality, judging by the parameters (temperature, pressure, etc.).
Xenon-133's half life is 5 days, and xenon-135's half life is 9.2 hours. As to whether the amount of xenon is large or not, the company is evaluating with the help of experts.
Neutrons that exist inside the reactor hitting uranium or plutonium
Curium-244, -242, causing "spontaneous fission"
(I missed the third possibility that Matsumoto mentioned.)
It would be difficult to deny that the same thing is not happening in Reactors 1 and 3, says Matsumoto.
He thinks the possibility of re-criticality is low with the damaged fuel melted together with control rods and other things. Even if it does happen as the reactor is cooled further and the amount of water increases, TEPCO thinks it can be controlled by carefully monitoring the temperature and pressure.
(Reporters seem to want to reassure themselves by saying "In a nutshell, even if it is in re-criticality, TEPCO can control it, right?")
TEPCO's handout for the press on November 2, the result of gas analysis in Reactor 2 is reproduced below. The numbers are still being evaluated, says Matsumoto: