Given the military-industrial-surveillance complex and that a moderately skilled hacker can construct one, they are no doubt in use by private security and intelligence agencies, as well, with G4S and Academi/Xe/Blackwater certainly coming to mind. TigerSwan, a competitor to Academi and G4S, (unsurprisingly) may need to be added to this list per the documents released by The Intercept. https://theintercept.com/2017/05/27/leaked-documents-reveal-security-firms-c... https://www.democracynow.org/2017/5/31/private_mercenary_firm_tigerswan_comp...
This evidence is not proof (there could potentially be other uses of the IMEI number or they could be collecting that to pass on to government entities that then IMEI/IMSI numbers through their catchers), but is certainly suggestive: https://theintercept.com/document/2017/05/27/shared-daily-intelligence-updat... (pg 19 of the 5 Nov sitrep)