走り去る姿でいる is semantically odd, since 走り去る expresses an action "to run away from a location", whereas 姿でいる means "to stay/exist in a location in the state of~". Those two expressions can't coexist together.
Oh, that makes all kinds of sense. Switching around from でいる to 路地に should be fine therefore to indicate the existence of the man in the alley as well as the direction, movement or destination of his action. I really need to get back here more frequently.
That is strange. Another Japanese friend told me it was not a mistake, although of course the normal phrasing would be
腕に血のようなものを付けたまま現場の近くの路地に走り去る姿が目撃 されている. Sorry for your troubles !