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There could be a few reasons for these IDs showing up in Google Analytics 4 (GA4) even though the website is not using Google Tag Manager (GTM). It’s possible that the site implemented GA4 without GTM and hard-coded its tracking capabilities in the site’s code itself. Alternatively, they could be using another Tag Management System, which is not Google’s. Also, there could be a server-side implementation where these IDs are being passed along to GA4. Basically, when a user logs into the website, the server could be sending these details of client ID, affinity ID, and customer ID to GA4 as part of the payload of the event API call. As for the lack of a GTM tag, it is possible that it was removed or never implemented, especially if the website did a direct integration with GA4.
