1. Why bother spend money on Lojack or ANY other security device and then hand the keys over to someone you don't know? Anyone that valet parks their car deserves for it to be stolen. Your "bro" made the biggest mistake...gave the keys away.
2. If you're suspicious that the Lojack unit was not installed, ask the installing company to prove it. I don't see why this is an issue for you, as your insurance company would be pressing the issue in a BIG way, as they would be looking to deflect liability. Call the Lojack tracking number, and confirm the car is able to be tracked.
If someone was joyriding an E55, it's unlikely they hot-wired the car. They probably had the keys.
1998 C230 "Black Betty"