Green’s Dictionary of Slang

to observe, to survey, to keep a watch.

1873		Sl. Dict. 368: Tommy Tripe to pipe; that is, to observe. ‘Tommy Tripe his plates of meat.’.
1890–1904		Farmer & Henley Sl. and Its Analogues.
a.1909		press cutting in J. Ware Passing Eng. of the Victorian Era 115/1: If a chap happens to be a dab tros he gets on better than a doogheno who keeps himself quiet and never lets anybody Tommy Tripe know how clever he is.