Happens to me constantly and it does my head in honestly. Record something in a room that sounds dead quiet, get home, stick the headphones on and there's all this layered noise I swear wasn't there. My last session in an old mill in Salford had what I can only describe as a low humming that definitely wasn't audible on the night, picked it up clear as anything on playback.
Whether thats the recorder picking up frequencies our ears just aren't sensitive enough to catch in the moment, or something weirder going on, I genuinely don't know. The skeptic in me says its probably HVAC or vibration from nearby roads but then why does it only show up on certain nights in certain spots.
Anyone managed to rule out environmental causes properly? Would love to know what kit people are using because I wonder if some recorders are just more sensitive and basically trolling us.