On some cheaper code readers its very easy to clear when you dont mean to. If all the monitors are not on, it wont smog until they are. Same if you have a dead battery or disconnect it. If there is an evap code active, and you disconnect the battery or it goes dead, it will set a permanent dtc which can only be cleared by driving, and i mean a lot. From cold,30 idle, tank has to be between 1/4 and 3/4 full, drive cycle, then 30 sec idle before shut off, probably 30 times. We have put 2500 miles doing that to clear them.