If you're serious about getting an ID on that spider, we're going to need more than a belly shot of a dead one. Flip it over, get a picture of the dorsal side, get a close-up shot of the eyes, and since you keep seeing them at work, try taking pictures of live one in situ - are they in a web? If so, what does it look like? Are they running around on the walls? Hiding under rocks? In dark corners?