This recon mission can be triggered by completing the Boston After Dark quest and by visiting the team of Railroad members who have set up a base at the Ticonderoga safehouse. Speak to Desdemona, who will tell you to check on the status of the safehouse.
The safehouse has been overrun by Synths. Your two mandatory targets are Coursers, but you'd might as well take out all the Synths for loot and experience. If you take the elevator up and work your way down, you should find every possible Synth and your targets.
Return to Railroad HQ to give Desdemona the unfortunate news.