That's actually a very good question, NancyDrew. And it really makes me think. Let's say a neighbor did see Alex leave and knew Allison was home alone. This is a very possible scenario. But how did they know he wasn't coming right back? He could have been making a beer run or quick trip to the grocery store.
Still it is a possibility. So taking your idea a step further, who else could have known Alex left for the night and Allison was there alone?
Who did Alex tell? Who did Allison tell? I think we need to start thinking more "inside the box".
He most definitely might have called one of his close buddies. She might have told her parents. Who else could she have told?
If the perpetrator lived around the neighborhood, and drove or walked by the house often, he could see that there were no cars parked out front, so Allison must be alone. Assuming the alarm was not set yet, the perpetrator could have walked in an unlocked door, and proceeded to attack Allison. Apparently there was quite a fight that took place in the house, and she fought back bravely. Unfortunately, it could have been a rapist from the neighborhood, as it seems from the limited information that we know, the perpetrator walked away from crime scene, probably through the alley.
The question is, who would know she lived there alone? Neighbors close by would, and as far as it being a random attack, it seems too risky with the security sign in the front yard for a robbery, especially with the lights on in the house. So IMO, it was either someone who knew her, someone close by, or someone who travels down Monterrey Way often, and has been paying attention to her.
There is always the outside chance a rapist happened by and saw her on her way into her house. This happened in Venice, California a few years back, where a Korean-American woman who lived not too far from a bad part of Venice was taking her groceries inside, and a man recently released from a mental institution followed her inside her home and tried to rape her, and when she fought back he stabbed her. I mention this since it is possible for things like this to happen, where random strangers walk the streets looking for victims, but in the case of Allison Feldman, it just seems unlikely to me that this was some random criminal.
The good news being the DNA they have from the fight, so there is certainly hope the killer will be caught eventually. If the man is not caught right away, someone so savage that could bludgeon a nice young woman in the head is bound to screw up eventually, be arrested, and have their DNA tested. So perhaps some of the only good news is that since Allison chose to fight back, her bravery might very well lead to the killer being caught eventually. I just hope it happens soon.