I can't think of anyplace earlier to add something in town, but if you want an in-town fight, maybe a fight by a couple black cultists (disguised as Legionairres until combat starts) show up after (main quest spoilers)
Esmerelda points you to Evelyn. Before the fight begins, one of them runs off to deliver a warning. Poof, that explains why Evelyn suddenly magically knows to take off after you talk to Esmerelda – there were cultists spying on you. Issue: What about some meddling co-op partner who might be hanging around Evelyn/the clinic?