Love this show! I've used a simple trick with a strobe light or flashing different lights through the open door leading to the open basement door, along with short bursts of fog machine. For a safer fog, in case some of your smoke detectors might go off, use a can or two of haze spray cans towards the open door. The chaos of the noise from sound effects drowns out the spray sound. Here's a sample of it from Amazon.
Bruce Taws
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