If the last GPS location was the gas station, that would mean that the car would still be there or that it was disabled at that point.
Someone has to know the GPS exists before they could disable it. They'd also need to know how to disable it. That doesn't seem to point to an opportunistic situation, but rather a planned and well informed situation.
So, who knows about the GPS, would be worth knowing. I'd assume that Toni knows about it. I don't think it'd be a stretch to believe the BF knows about it. I could see co-workers/friends being told that. Maybe one of the regular patrons at the bar, whom she has friendly/chatty conversations with?
Has it been verified that the last known GPS location was the gas station?
It's possible that she was kidnapped at the gas station and taken via another vehicle. Then the attacker returned and disabled the GPS via Toni's knowledge of the GPS and how it works, then drove the car away and hid or disposed of it (body of water)
But, if the gas station was truly the last location of the car, it seems more likely that the GPS was turned off by someone who knew it was there and knew how to turn it off.
Does anyone have one of these insurance based GPS systems and know how you'd go about turning it off? Is the driver able to flip a switch or does it involve going under the hood or something more involved?