While I hope that is true, I don't take it for granted that LE theories are based on hard evidence. Often in cases where they really don't have much to go on, they will go with what seems most likely via the evidence/scenarios they do have. This feels like a theory based on the logical consensus that the area of her home is not a place a random stranger would target. Alas, the world isn't always a logical place, and there are plenty of cases where serial offenders have gone out of their way to find just such areas to target. Of course, the former scenario is solvable, while the latter is...not so much. Thus it's best if they continue to dedicate the majority of their resources on the solvable scenario, while keeping an open mind about the not so solvable ones.
As for questioning every worker that's been through there...well, probably the ones who work for the city or major delivery services that would have records of their destinations during work, but to go and check up on everyone who might have been there, such as taxi drivers, smaller delivery guys (think pizza, rent-to-own etc), animal control & such (ones who patrol, no record of where they go), plumbers and the like...well, that's too much for any LEA to be expected to track down, no matter how big the case, IMO.
Then again, I'm a hope for the best, expect the worst type, so I'm going to root for LE to soon nab a perp who has Sierra alive, while maintaining very pessimistic viewpoints on certain things. So, yeah, take my cynical thoughts with a grain of salt.
All JMO.
