I think it's reasonable that the perp was hoping she'd be picked up by the trucks. I wish I knew more about what prompted the family to go look at the tarp. Perhaps the truck had JUST gone past and they were attending to this because it was missed and not picked up?
Given Bar2121's estimated time window for her trash pick up two streets over, the perp could have left the bundle late in the evening, after seeing that the trash hadn't been picked up yet, and then, because of the unusual location of the bundle, the trash truck left it, at which point the family who discovered it went out to get it out of the street.
The tarp was up against the curb on the side of the house, it wasn't out front. The section of Cindy Lane where Alanna was found is lined with fences. The side and back yards of houses on both sides are oriented toward Cindy for the entire block. There are no houses that face that section of road. (I know this from walking around with Google Maps) it just looks like a long corridor lined with fences. So, it would have been a mistake to leave trash there, hoping for pick up but it would also have been easier to quickly dump the bundle containing her body out the passenger door at the stop sign without being seen. Which could be why the perp dumped her as close to the corner as possible.
I described in a previous post how easy it would be to slide her out at the stop sign. It would have been very quick, and virtually invisible.
ETA: locals have said the trucks will pick up bundles either day. They apparently don't enforce the second day rule very strictly.