Are you referring to a general suspicion you have, or a specific case? Because this is categorically illegal, and has been punished before [1] to the tune of $415 million.
[1] https://www.latimes.com/business/technology/la-fi-tn-tech-jo...
None of the companies are explicitly getting together and saying "let's keep pay at this level", but there's definitely some fishy stuff going on.
Totally outside of the HR realm, when I was a computer system product manager pre-Web, we had a consulting firm that we paid way too much money to and when we were going to have a product launch, we asked them to, yes, fax us data sheets on various products we were positioning against. Later, there was another firm that collected pricing information which we needed for our pricing proposals.