I have been discussing this recently with some Prius buddies. The question has come up whether or not OBDII equipped cars can record significant events and those events can be viewed by the dealer technicians when you take it in for service.
So my question is, can OBDII register things like adding the EV button to the prius and using it. It may void the warranty if they could tell. Also, if you add batteries that might also void the warranty. Also if you have an HCH and like to key off and back on for better FE. Can OBDII record things like that and report it to the dealer tech?
I would say that it's unlikely that "significant events" would be recorded unless you're getting a check-engine light; however, I know that GM has implemented a "Black Box" system on many of its vehicles that records a vehicle's speed, braking action, and other factors in case of an accident, to assist in an investigation of the event. A bit of the "Big Brother" effect that most people didn't/don't know about. If you have a ScanGauge, do a quick code-pull to see what you get (sometimes out-of-range values will be stored, but not illuminate the CEL). Then you might be able to clear selected items.
Unless you're taking it in for warranty-related issues and you need that stored CEL code, it would probably be a good idea to clear the ECU for routine checkups, just in case. I'm thinking that theoretically this would be a separate logging system from the diagnostic (OBD) system. If you can't get a tech that works on these to give you an answer, I'd say brainstorm as a group to see what the vehicle would recognize the EV button as being, for example -- "Malfunction, Synergy Drive Sys, Engine not Charging Batts".
Just an idea -- I was always paranoid of Mitsubishi when I had my EVO. They would send spies out to autocross and racing events to secretly record the license plates and VINs of cars, then send them to the central DB where you're warranty was voided: "The vehicle shall not be used in a competetive event..." Meanwhile, Subaru was giving away SCCA licenses free for a year and wanted to promote their vehicles' abilities and market that lifestyle. Sheesh.