Could be knock. Not all runtime errors are recorded and from what I understand some require a certain threshold to be surpassed in any given instance before it's recorded. There is no code for "OMG there's a hole in the #1 piston and your rods are a knockin!". You can try driving around with the diagnostic mode set and see if any additional codes show up. Naturally there will be a code 51.
Power loss at the ECU to the point of clearing the codes is a possibility but usually this isn't consistent and you'll see a code for inconsistent power show up sooner or later.
Code 51 is just indicative of the IDL switch being open the A/C circuit reporting as "on" and it clears as the trigger condition is resolved. So, it'll show up it you step on the throttle, have the A/C on or the TPS is misaligned but once the condition is remedied it'll go away after a few seconds.
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