Marcus marvels at Polis when the crew arrives for the negotiations in episode 3x03 "Ye Who Enter Here". Lexa battles Roan in episode 3x04 "Watch the Thrones". Octavia talks Indra out of her funk in episode 3x07 "Thirteen". Appears in 1 more episode.
Thea gets into Merlyn's limo in a flashback to right after the attack on the city in episode 3x03 "Corto Maltese".
The Fringe team investigates a bunch of bodies that have washed ashore in episode #209 "Snakehead".
Sam Flynn's arrives at home and is visited by Alan Bradley.
Tinga is chased by Lydecker's team before being saved by Max and Zach in episode 1x12 "The Kidz Are Aiight".
Frank's car is pulled from the river with his body in episode 1x01 "Pilot".