Oh! I just realized a mistake in the HTML build. The instructions aren't updated in that version, so it doesn't tell you how to sprint (Hold CTRL/SHIFT to Sprint). With sprinting, catching up to the intruder shouldn't be near-impossible.
(another tip is to place tripwires in locations the thief is likely to cross!)
EDIT: I've just added the instructions for SPRINTING in the description below the game.