Fuel Octane Issue? - 2003 Prius

Periodically, we get a check engine light after going through about 1/2 tank from some "unknown" service station, usually on a long trip somewhere. Light stays lit until shortly after next gas fill-up, when it extinguishes and stays off (perhaps for many months) until the next "rouge" refueling. At about 35,000, dealer replaced catalytic converter due to this issue, but problem returned soon thereafter. Now, dealer says anything from "fuel door not tight" to "bad gas" (but won't tell me the codes). Having grown tired of paying $95.00 each time they run analysis and clear the codes, we recently purchased our own OBDII, but problem has not yet recurred (but it will). Question is, has anyone experienced an issue like this, and/or have an opinion concerning alarm conditions possibly associated with "bad" fuel (no, we won't use an ethanol mix). We have come to ignore the many warning light episodes, and that's probably not too smart. Thanks and this is my first thread. 2003 classic; 81,000 miles; 45.6 in winter; 47.5 summer.