if there is no customer ID sync on your site, then the customer ID volume must be coming from another site or app, or it is coming from offline data
to check for offline data, go to analytics > Onboarding status report (you need aam admin flag to access this report), then check the customer ID data source
to check which website / app could be the source, first check which of your traits are populating with customer ID volume (that should be the main clue). If that doesn't work you can go to audience data > signals, and do a search something like this:
d_cid_ic contains XXXXX
AND
c_pageurl
Where XXXX represents the integration code of the customer ID data source. This will then show you which urls are populating that data source. If that doesn't work, you can try with other common parameters (c_channel, app ID etc) until you work out the source.