Disconnecting the battery will not clear the codes or put out the light on cars with OBD2. That's because for many years ECU's and OBD systems have used E2PROMS (sometimes written as EEPROM, Electronically Erasable Programmable Read Only Memory) and so loosing the feed will not erase, it needs a signal from a code reader kind of device.
As for lights going off themselves some fault codes are classified as lower priority and a number of cycles (often start, run for x min, drive above x MPH, switch off) are programmed in and turn off the light but will not delete the code (in some cases).
Its all a bit of a muddle, my BMW Motorbike had OBD and a 5x drive cycle would put out the ABS light but only for the low voltage fault code!
Best advice is as above, buy a code reader as it WILL happen again and once you get the hang of using it its 30 seconds to read and clear the codes. My last +4 (2008 Duratec) regularly put the MIL light on as Graham can attest...... as he has seen me do it on many occasions ;-)….. and heard the bad language!
BR
Colin
