The exact location of the purse (on Laguna between the metal shack and the cactus) gives us a little more insight into what has happened.
That was a "pretty good" place for the perp to put the purse if he didn't want if found "any time soon". If it was some sort of "staging", the perp would have left it more out in the open. Of course, there are better places to have hidden the purse. He could have taken it to where ever he was taking Sierra. Purses are easier to hide than bodies. I think he just wanted to get rid of it quick. Since, presumably, he had Sierra with him, it doesn't seem like the purse was all that important. He could have been concerned that a second phone or other GPS was in the purse and pitching it was easier than going through it. He might have been concerned that something from the purse might fall out on to the floor of his car and later be used to link him to Sierra.
It is surprising that he was driving on Laguna at all; it is a cul de sac. There is the possibility that his destination was east on Laguna. Hopefully everyone who lives there, all structures and farms etc were thoroughly checked out.
It is possible that the perp turned right on Laguna because he thought it would lead to the Freeway or Monterey Rd. Hopefully people on Laguna were questioned about a strange vehicle that morning.
It is possible that the perp used that location to either better secure Sierra or actually remove her clothes or sexually assault her. It seems a little too exposed for that and the dogs detected no scent and there was no other evidence.
There is the possibility that the perp returned with the purse later after news of the extensive search got out, to deliberately allow it to be found. This would be extremely risky. Police and residents would be on the lookout for anything out of the ordinary. Also, there does not seem to be anything to be gained by that sort of staging.
The most likely explanation I can think of is that the perp wanted to get rid of the purse ASAP and there was no vegetation or other suitable place along Sta Teresa to pitch it where it wouldn't be quickly noticed. There may have been some traffic along Sta Teresa and he didn't want to be noticed.
It occurred to me that he might have dumped the phone and purse along a northerly route in order to set a false trail north while he turned around and headed south. If this were the case, the purse would not have been hidden so well. I think his planned destination was to the north.