Continuity mistake: When Blade has been rescued by Abigail and Hannibal and the 3 of them are approaching the hideout, notice in Abigail's back quiver there is one arrow left, yet in the next shot when they're walking up the stairs she has two.
Continuity mistake: When Blade jumps through the police station window and lands on the pavement outside, we see a lot of broken glass from the window falling with him and landing all around him, yet when he stands up and says, "Forgot my sword," there is no glass to be seen anywhere.
Continuity mistake: Hannibal wears a "Hello my name is" tag that seems to slightly change condition and placement during the rescue of Blade from the familiars. Then when they get back to the Nightstalker lair, there's a close-up in which the tag looks nearly new.
Answer: It's possible that he followed one of them back to the base. This is feasible since they wouldn't be able to tell that he was following them because of his shape-shifting abilities.