You can’t detect such a high frequency noise with your phone alone. Also, by saying “flat line” I understand you haven’t analysed the harmonic content of that recording, but instead have just checked its waveform. This is not how you properly assess this, as your phone isn’t capable of catching such a high frequency in the background and you wouldn’t be able to see such s high frequency by checking the waveform, especially if this sound is very low in volume and quite hidden. There are many biases involved: one above all is the mic built into your phone, which for its purpose has a reduced frequency range it’s capable to catch. You’d definitely need a professional microphone for these kind of situations and analysing that recorded sound with the help of a proper audio editing tool, like Adobe Audition and use its spectrogram feature to properly analyse the sound.

You definitely should ask some audio professionals to assess the situation: make a proper recording of that sound and analysing it using a sound spectrogram. This is how you’d be doing in order to understand whether that sound is actually there or not.