It's hard to diagnose from the photos, but as you noted, it appears that the lower stem has been chewed. Culprits include rodents (especially since you said you already saw a mouse in that area) or an insect like Black Cutworm. (They feed at night, burrow in the soil, and can also feed below soil surface.) If you're not finding any active, they might have escaped detection or even been eaten by a predator (birds, or even the mouse getting an opportunistic snack). Management information is included in the linked page. Since you didn't see damage to germinating corn, this is less likely, but another pest of young corn is Seedcorn Maggot.

Miri