Visit type returningCustomer w/o commerce setup

I’m curious how/when visit type is recorded as ‘returningCustomer’.

I have a site whose new and returning segments weren’t adding up to the total visits. I found out that some visits are being recorded as returningCustomer, and that seems to fill the gap. And although the site does indeed sell products, we’ve never set up the Ecommerce portion of Matomo. All Ecommerce reports/logs are zeros.