Home for the Holidays - S11-E1
Other mistake: Only one set of prints were said to be on the garrote but in the last episode of season 10 Graham handles the garrote before handing it over to the murderer without wearing gloves. So there should have been 2 sets of prints.
Suggested correction: The show is set in the early 1900's, which is the early 20th century. By the time this event transpired, SOS would have been established as a universal distress signal.
The show starts in 1895 and by season 2, it was still the 19th century and before Germany adopted SOS in 1905.
Bishop73