Thanks for all the information--Flyertalk is a font of knowledge. We were never able to get through to CBP and they haven't returned phone messages, but I was able to verify things thanks to you all pointing me in knowing what to search for. (And no, it's never too obvious to suggest checking to see if the form is stapled to the passport. Those are the kinds of things one misses when in a slight panic.)

The upshot is that Minneapolis recently shifted to electronic ESTA clearance (they still used the paper forms in April) and it was the I-94W departure form that had been expected and wasn't there. There is a notation in the passport itself with her anticipated departure date, so apparently all is good.