I believe the HC500 has visible illumination so it is definitely visible to critters and humans.
A mistake us humans tend to make is judging action/reaction by animals based upon our limited senses. Just because we cannot sense something doesn't mean critters cannot. I read where the HC600 was "covert" and I researched it and others thoroughly.
What I found viewing many chips is that "none" were invisible in the sense critters knew they were having their photos taken. Reaction to the cameras were varied over a long period of time.
The HC600 is basically invisible to humans unless one is right next to it. During my research I viewed a clip of a coyote that turned, looked at the HC600, walked over to it and gave it a good, once over, all the while having it's mug taken, then walked off, not disturbed in the least.
I have enough experience with the HC600 to know that deer and other critters know its taking photos, but seem to have little issue with it. They continue to walk, feed or browse past with little more than an acknowledgement that it's there.
The following is the fourth photo of a grey fox that stopped when the HC600 took it picture. The series shows it turning, walking up to the camera and in this final photo is standing up and checking it out.
I have 5 photos of the same large, male bobcat, on the same trail, walking past the HC600 at less than two feet and paying it no attention. Deer routinely look at the HC600 but ignore it.
My conclusion with the HC600 is that critters are seeing or hearing stuff we humans cannot, but whatever it is doesn't seem to bother them.