I've never seen something like that. Do you lay the CANHI and CANLO wires along the circuit board and it detects the signals? That looks more like a capacitive coupling. In dominant state your would get a differential voltage on the two capacitors In recessive state they would be at about the same voltage. I would guess it's a FET input amplifier and maybe a comparator to make the signal digital.
The best way to monitor CAN is to electrically connect a CAN transceiver to the CAN lines. On vehicles there is almost always a connector that can be used to connect to the CAN bus. I am less familiar with CAN in industrial settings, but I would guess that a connector or terminal strip would be very common.
You could do it with an inductive sensor, but you'll have to separate the wires and only sense one. If you put an inductive coil or sensor around both wires you won't get any signal.
CAN has about 3V differential in the dominant state, so with a 120 Ohm termination resistor you will see around 25mA of current. That should be detectable with a suitable inductive pickup. The current will be in different directions depending on which side of the sensor the transmitting node is located, so you will want to make your detection circuit respond to either polarity. In the recessive state the current will be very nearly zero so it should be pretty easy to differentiate.