DC buss drops to 13.4 volts

I've been coming here since I got my Prius in December of 09. Up till now, I've been getting my answers by searching. I've learned a lot. This time, however, I don't see my problem.
For the most part, my dc buss stays at around 14 volts but lately, I've noticed the voltage at 13.4. I notice this after pulling into a parking space. The car is still ready and in drive and it stays at a solid 13.4 volts. If I shift to park, the voltage jumps back up to 14 -- put it back in drive (or reverse) and it drops to a solid 13.4. If I shut down and go back to ready, I have a solid 14 volts and can do nothing to get it to drop back down to 13.4. This bug shows up when it feels like it.
The next time I noticed the 13.4 volts, I fooled around and this is what I was able to tell:
o The car has to be in drive or reverse for the voltage to drop. o AC auto must be off. Switching it on brings the voltage up to 14 -- off and it drops back down.
o Radio or headlights have no effect.
o Rear window defogger brings voltage down to 13.2 volts.
o Driving does not bring the voltage up. It stays at 13.4
o Shifting to park or neutral brings the voltage back to 14.
o Turning ac auto on brings the voltage back to 14.
o Shut down and restart and I have a solid 14 volts no matter what I do.
Voltages are via scan gauge, volt meter at jump terminal under hood and MFD (signal test). They all agree within .2 volts. Battery is at 12.2 volts after overnight rest and before powering up. I removed it and charged it for 36 hours at 2 amps. The battery terminals and body connection for negative cable are clean and tight. Sorry, I don't know what the voltage was right after charging but for a while, I'd see 12.4 volts before powering up.
If I could reproduce this problem (?) at will, I'd bring it to the dealer but every time I'm in the neighborhood, the scan gauge says 14 volts! O yeah, I tried disconnecting the scan gauge and that did not help.
Any suggestions or advice?

 

I'm a little scared to disconnect the battery while the car's running. I won't see any sparks, will I?

If I do disconnect the battery, I'd probably just undo the bolt from negative cable to body. Does that sound right?

 

What you describe pretty much sounds like me. I didn't think to try running just the fan to see what effect it would have. I did try the front defroster but that turns on auto AC which brings the buss back to 14 volts.

It won't be long till I have the AC on full time and this won't be a problem till fall. I really didn't notice this low voltage till about two weeks ago. Now I'm going to be paranoid about it. Maybe it will drop when just the right combination of conditions exist even with the AC on.