Do you think they might be putting in random HF content just so it looks like there's something there?
I can't speak or vouch for anybody else; I don't.
But it is hard to avoid the noise from the venue; below 20 Hz, there is likely to be ventilation/temperature air conditioning gear noise, Then there is what I have ultimately termed Tesla's Greeting from the Grave : mains frequency (and the whole myriad of its harmonics) hum and buzz - the easiest way to tell if it is an european or an american recording...
Above 20 kHz ( and slightly below, beginning around 19 kHz ) , there may be movement detectors, various lighting "accompaniement " , an ocassional insect ( ah, those charming moments in the quietest passage usurped by the close flyby of a mosquito/fly/bee/wasp/whatever ... ) - and who knows what else is lying in ambush, waiting to jump me in the future.
As I record in DSD - and almost exclusively on location, preferably live - I can only check for this LATER - as it requires the DSD to be converted to PCM in order to be able to use spectrum analyzer such as Voxengo SPAN. I would love to have the time available to set up an additional PCM recording rig to see what is going on right away - but given the real world time constraints, I will always put microphone placement as a priority - and not > 20 kHz " environmental pollution ". And when recording acoustical music, it is relatively easy; anything that requires electricity for the musicians to be heard, may well be booby trapped with various switching power supplies - which, once assembled in the actually used system, can have nasty ultrasonic output.
There is at least one soundcard that takes the "juice" for its phantom mic supply off the main switching PS - producing, as you might have guessed - a small, but distinly defined spike > 20 kHz; in whatever it tries to record, from the earliest possible stage...
Now, imagine a multichannel multimiked recording ... using say at least three mic preamps from at least two manufacturers. Unless the equipment is not strictly vintage ( with decent and solid linear power vsupplies ), all those switching power supply frequencies are bound to find their way trough all the cabling, one way or another. Even in balanced XLR connection, there is something called common mode rejection ratio - CMMR - and it definitely is NOT infinite; some of the noise ( including > 20 kHz ) WILL come trough .
So, above 20 kHz spikes and other noise not related to music itself most likely it is not intentional, but probably next to unavoidable in real world situations. And is something to be added to the "watch for and avoid" list for the future recordings.