Hidden Interference

[Tom Koerner]

I was shooting on a street around 3am. I had on my trusty Sony MDR-7506's and I felt I had fairly strong audio. I know I was paying attention because I did some extra takes expressly for SFX. When I wrapped and reviewed the footage, the audio had some very gnarly interference.
I suspect one of the following: neon "Pizza" sign on the window relatively near the shots, fluorescent lights inside the building nearby (unlikely), or the streetlamps. The odd part of this is I have shot all up and down that same street for various reasons, and never had that same noise before. BTW the mic, Panni shotgun AG-MC15P, was mounted on the camera with a 3 inch cord.
Any thoughts on the culprit would be nice, but I'm more interested in why I didn't detect it on the headphones. An audio guy far more competent than I recently had the same problem, so I'm fairly certain there is more to this than I suck.

[Douglas Spotted Eagle]

You very well could have RF, you might not have heard it if the light had loud enough noise that you were hearing it through the phones as well as in the phones.
How loud is it? I'd be surprised if Noise Reduction from either Sony or Bias couldn't kill most of it if it's constant.
Are you sure your mic cables have no broken shield? Open ground?

[Tom Koerner]

Thanks for the suggestions! The cable shield is intact, i don't know what open ground is... And i'll bet noise reduciton would have been great. I ended up using a noise gate and looping a lessened track of the intereference to lower the obnoxiousness slightly. It was still pretty bothersome, so I redid all the audio sans voices in a foley pit (the sidewalk), dropped the loop, and gritted my teeth all through the little dialogue there was.
I guess my main question is - how would you normally deal with this? My headphones are closed (at least in theory) Are there any sort of tests i could implement? Would flagging the mic be futile?