I finished reading the helpful links posted. I agree with much of what was suggested, but there are still a few things I'm not clear on.
I'm Briana's age (not age she was when she went missing, but age now), so I was 17 when she was 17. From what I remember, cell phones weren't popular for those our age until maybe a year or so later, and even then they weren't that common and weren't used as frequently. I know I didn't discover texting, for example, until I was 19, but that could have been different for those whose parents caught on to technology a little sooner than mine did. Anyway, where I'm going with this is, has it ever been confirmed whether or not Brianna had a cell phone? Judging based on her circumstances (her mostly being independent from her parents), I would doubt it. I ask because I'm still wondering if there was any way to see whether or not she had planned a potential meeting with somebody or whether anyone was informed when she was getting off work.
I initially thought that the most likely scenario was that someone had climbed into the car in the parking lot and was waiting for her, and I based this mainly on the short period of time between her leaving work and whatever happened. However, I'm starting to agree with how unlikely that might be, after reading a couple of the links above with other opinions.
I really have a feeling that Brianna planned a meeting at the farmhouse, which is why I'm wondering if there was any way to verify who she'd been talking to. To me, this also may explain why she really declined to stay after work. Unless the incident happened before she got to the farmhouse, Brianna obviously went there willingly, so I am inclined to think that the reason she didn't stay had less to do with her being tired and having to get up the next day and more to do with her having already made plans to meet at the farmhouse with someone who knew what time she was expected to be off work.
I guess in 2004 teens didn't use texting or social media the way they do today, so plans like this would be more difficult to trace. I guess if Brianna did plan this meeting with someone they would have most likely planned it in person, although I wonder, since she didn't go to school, where she would have talked to someone. Am I making any sense? I mean obviously she had friends and knew people, but arranging meetings like this seems a little more difficult if you don't have a cell phone, don't use the internet for commnunication as much as we do today, and don't go to school, so maybe this theory is unlikely, too. However, those she hung out with regularly likely knew what time she usually got off work, so I guess it wouldn't have been too hard to just tell someone to meet her at the farmhouse. I know she told her roommate she would be home that night, so I'm not suggesting she had any big plans, just that she was meeting someone quickly to pick something up or do who knows what.
Anyway, what I'm getting at is that I, like most others, think whoever was involved knew Brianna as opposed to being strangers. If she didn't plan this meeting previously, another option that came to mind was that maybe this abandoned house was a frequent meeting place for her group of friends, and she happened to see some people she knew when she drove by. I doubt this would be the case about the building being a hangout, since it was apparently clearly visible from the highway.
I just don't think it's likely that she would have stopped at the farmhouse for a stranger, and as others have pointed out, the hiding-in-the-back-of-the-car scenario would need to involve a getaway car, as well, which I guess isn't impossible but is unlikely. My guess is that this was a pre-arranged meeting. I just wish we could have traced her communications the way we likely could today.