Let's try this....
~HYPOTHETICALLY~trying to make it all fit.....
Having NO IDEA where Kelly is, how would this mystery person be sure they could get into Kelly's house overnight on Jan 15th or the morning of Jan 16th to leave the doors wide open without Kelly herself popping up? How would he know she didn't just take off and whether or not she'd just choose that moment to come home? The only way you'd know that is if you knew what happened to her and that she wasn't coming back. And he would have to know that LE wasn't coming back at that time...or family. And wouldn't it stand to reason he would go in through the back to avoid detection? I see french doors on the rear of Kelly's house on Google. If so, he would have to know the back of that house and the back yard.
I think he lives in her neighborhood and is close enough to have an unobstructed view of her property from his. The only thing random, in my opinion, was the end result. I think he was sitting there outside BluePrint. I think he did that a lot. Maybe she knew him in passing, assuming that he worked at BluePrint or nearby, because she saw him outside so frequently. I'm guessing that perhaps he was waiting for her next break? Not to take her, but just to observe her, expecting her to be with other employees for her walk. But this time she came out alone.
Why do you drop the gloves and the goggles on the ground at BluePrint after the fact? I'd imagine that first, they were an oversight and got left in his car. Second, I would think he could have dropped them there to add to speculation that Kelly never left the area. He wouldn't draw attention to himself by searching. He's just a concerned neighbor.
I don't know why Kelly left the building the way she did. Who knows...it's flu season, maybe she wasn't feeling well and needed air? Maybe she ate or drank something that didn't go down right? Too much coffee? Early onset menopause, frustration, boredom, there could be a million reasons.
I think he would have to be parked at or near the dead end. If so, and he somehow managed to bait her over, all he would have to do was get her on the opposite side of a large vehicle. Then she's out of sight of the building. I think it's highly likely that a vehicle could be parked there at the dead end and no one noticed it, especially if it had been there so often it was just part of the scenery.
This isn't someone walking a deserted country road. This is a dead end with only one way in or out unless you're in a boat. One random thing had to set all of this in motion and for that to happen, the rest of it had to already be in place before she darted out the door at a time other than her usual.
He was already there.....waiting....... he just had to be
***ETA....I say "he" and that's only out of habit...