Is leaving a Teflon rice cooker on overnight dangerous?
There is no risk to the Teflon itself, or from it; PTFE breaks down at about 500 F, which rice cookers will not achieve, especially on warming setting.  Other than that, PTFE is one of the most inert substances known to man, as the atoms are already bound in energetically very favorable bonds:  little is as able to displace them without significant input energy (thus the high breakdown temperature).

On the other hand, you should not hold rice for a long time without either refrigerating it, or having it above 140 F (60 C) as there is some risk of Bacillus cereus growing and causing food borne illness.