Toilet tank leak - no apparent signs.

[Terry]

you assumed that water from the overflow went to the bowl

Yes. This is what refills the bowl after a flush. If the fill tube is not directed to the overflow tube, the bowl doesn't fill completely. The bowl can't overfill; any excess water goes over the top bend and down the drain.
Try slowly pouring a bucket into the bowl, you can't fill it higher then the top bend.