Martha Jones has crossed over from 'Doctor Who' to 'Torchwood', making her one of the few 'Doctor Who' characters to appear on other shows. (Captain Jack Harkness, Sarah Jane Smith, The Brigadier, K9, Toshiko Sato, and the TARDIS would be in that group as well. Maybe there are a few others....)
And she's been a busy girl since she left the Doctor's side - she's now a medical officer assigned to UNIT.
We got to see her UNIT ID badge in "Reset", the episode which introduced her to Team Torchwood in Cardiff, and her serial number is "001 23 14 01 2 1".
"23" is part of the numerical sequence from 'Lost'. "01" & "14" would equal another in the sequence, "15". And the last three numbers add up to "4", which leads off "The Numbers".
Definitely a stretch, but we've seen far more trivial theories in Toobworld.....