No point to have them at a station like Grosvenor. Most operators make it very obvious the train is going out of service and some even make announcements along the lines of "If there are any passengers still aboard this train, please remain seated. It will return to the Grosvenor station momentarily" as the train pulls into the pocket.
If you haven't gotten off the train after announcements like these (from different T/Os), I would be very surprised:
Entering Medical Center: "This is Medical Center, doors opening on the left. This train will be going out of service at the next station. Customers to Shady Grove, if you do not wish to wait outside, please get off here and wait for the next train to Shady Grove."
At Medical Center: "Red Line train to Grosvenor, next stop Grosvenor, last stop for this train, thank you for riding MetroRail and have a good morning."
Leaving Medical Center: "Grosvenor, next station, and is the last stop on the Red Line for this train. Grosvenor, next station, and is the last stop on the Red Line for this train."
"Next stop, Grosvenor-Strathmore, the last stop for this train. This train will be out of service at the Grosvenor-Strathmore station. Please be sure to take your personals, and newspapers, and thank you for riding Metro."
"Next stop, Grosvenor, last stop for this train. THIS TRAIN WILL BE OUT OF SERVICE AT GROSVENOR!!!" (flashes lights on and off)
Arriving at Grosvenor: "This is, Grosvenor-Strathmore, the last stop for this train. Doors opening on the left side. This train is now out of service. Please be sure to take your personals, and newspapers, and thank you for riding Metro. Next train, 4 minutes!"
"This is Grosvenor, last stop for this train, doors opening on the left. (turns lights off) This train is now out of service. All passengers must leave the train. Thank you for riding Metro and have a great day."
At Grosvenor: "This train is out of service. All passengers please leave the train at this time. Do not attempt to board this train, this train is out of service. No more, no mas, all finished. This train, is out of service."
You think a sign like at L'Enfant Plaza is needed after some of those announcements?