Ernestus is patrolling the streets of Lower City. He can be found at X: 43 Y: -71, arguing with a group of Baldurians
(Saer Frunilla, Wuloona Dor, Porter Juxon, and Bliggerillo). They are slandering aloud the contents of latest issues of the Baldur's Mouth Gazette and blaming the newspaper (and Gortash as well) for sweeping under the rug obvious crimes that happen in the city.
Ernestus's reaction to the player character might differ depending on the positive / negative article printed about them in the Baldur's Mouth Gazette (or neither printed at all), as an outcome for Stop the Presses quest. Also, he might notice if the party try to break in and enter any restricted areas nearby (e.g., Old Jol's house or the backdoor of the Nortale's Hostel), and react accordingly.
If Gortash is defeated, Ernestus and the rest of the group disperse, and cannot be found or interacted with again.