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It is quite possible to take a GA tracking code for one site and put it on another site. This has happened to me quite a lot where people have lifted content or copied code from my site. You can see the hostnames in GA (you have to dig for it), which could explain this.

Ghost/spam traffic is a real problem for GA. UA codes are public and can be targeted with spam referrals or simply randomly hit (especially for UA codes that end in -1).

Filtering spam and getting useful data on GA is a never ending job that Google keeps making harder. (re removal of Service Provider / Network Domain [1])

[1]: https://support.google.com/analytics/thread/27808046?hl=en

IIRC the measurement protocol can be used to send fake traffic by bad actors.