This happens to me all the time and honestly it's one of the weirder parts of doing field work. I'll be standing in a room, recorder running, and I'll hear something faint - like a low hum or maybe a word - but when I play it back later the audio captures something completely different. Not just clearer, but a different sound entirely.
Had one session out near the Sandia Mountains a couple years back where I swore I heard tapping. Playback had what sounded like a drawn-out vowel sound, nothing like tapping. Same recording, same moment.
I've gone back and forth on whether this is our brains filling in gaps in real time versus the recorder picking up frequencies our ears literally can't process properly in the moment. Neither explanation fully satisfies me tbh.
Anyone else been keeping notes on how often the two versions match versus diverge? I started logging it and the mismatch rate is honestly higher than I expected. Would love to compare notes with people who are being systematic about it rather than just chalking it up to "spooky stuff."