The phone is gone. The killer took it. No room for doubt there. PTL was the last ping.
Surely the killer would've wanted to see what was on the phone. Check her last calls.
Some phones can be set to code lock except when receiving an incoming call. Then they either lock immediately when the call ends or allow a small window before locking again.
Did they call HE's phone to open the code lock?
They knew her phone would've been traced to PTL
They couldn't take the phone with them when they left.
They could've made a quick decision to call her cell with SM's cell in order to get the recent call list, etc. before they turned it off or removed the battery, (the possible gain worth the risk) planning to explain it away as HE harassing SM.
When they saw she'd called her roommate (whose name would've displayed- not the number), they would've known she likely told about SM's earlier call. So...damage control.
They could've decided SM would say he only answered once to tell her to leave him alone, not counting on surveillance catching him calling her number from pay phones.
STILL don't know why they didn't use a pre-paid burner phone, but anyway...
Did they leave fingerprints, hair, other DNA while retrieving a phone, purse, or something else from HE's car?
This would explain why it wasn't immediately visible to LE.
HE's DNA, even blood at the scene wouldn't confirm murder,
or specifically confirm the murder occurred at PTL.
LE did not have to announce where the murder took place. They must be 100% sure of it. They didn't have to say anything about the location at all. Was this an intimidation factor, telling the killers "we know exactly what happened and where?"