Not sure if they still do it, but they also used to put hidden / protected files on any and all disk volumes as part of their copy protection scheme.
From memory, in addition to sticking some pseudo-randomly named files in /System/Library, /Library, and -/Library, it would place a file in the root directory of all hfs+ volumes with xattrs set to hide and write-protect the file. Installers would then look for these files to check licensing status.
At the time, this was a fairly common trick with pro/prosumer proprietary software.
[0]: https://en.wikipedia.org/wiki/FlexNet_Publisher#Issues_with_...