From start to finish...

It happened on the premises of the highway police, where the driver was checked with his vehicle.
Through inquiries with the highway police officers, we know that Arthas was there. During the inspection, he jumped onto the trailer once. The police then put him down and did not know where he went afterward. He must have jumped back on unnoticed.
From the GPS data of Arthas’ tracker, we know that his involuntary journey must have started right after the inspection ended, at around 3 a.m.

The GPS tracker from the brand Tractive is white and orange. The tracker is enclosed in a transparent white rubber case, which is attached to the collar. At the time of disappearance, the battery charge was only about 10%.
The tracker was supposed to send a location every 10 minutes and still had battery and network connection until about 11:30 a.m.

However, only 5 GPS locations were sent:

• 02:52 – Starting area, highway police parking lot, 63768 Hösbach

•  03:02 – Highway A3 near 63856 Unterbessenbach

•  03:12 – Highway A3 exit 64 Rohrbrunn

•  03:23 – Highway A3 rest area Spessart Süd (Rohrbrunn)

•  03:42 – Highway A3 shortly after exit 65 Marktheidenfeld

One possible reason for the lack of further GPS signals is that the signal might have been too strongly shielded, depending on how Arthas might have hidden under the transported car.

The collar is made of white fabric and is reflective; it has a silver metal buckle. The collar hangs very loosely so that Arthas can get out if he gets caught on something. Attached to the collar is a transparent white rubber case, which holds the GPS tracker.

We can only guess how the driver proceeded after the last known location, Germany, Autobahn A3 exit 65 Marktheidenfeld.

We only know the driver's start and destination: 

the Netherlands to Bulgaria.

• The driver may have switched to the A6 near Nuremberg.

• It is likely that he first drove through the Czech Republic, possibly Prague, and then continued through Austria into Hungary.

• It is also possible he took the A3 via Passau through Austria.

• Possible first stops include the German regions of Passau or Nuremberg, the German-Czech border area, the Czech regions of Pilsen or Prague, or Austria.

• If Arthas traveled for a longer time, he could also have ended up in Hungary, Slovakia, or even as far as Bulgaria. 

There are no records of license plates, vehicle descriptions, or the driver's identity.

1. The exact route and the order of the countries crossed are unknown.

2. We cannot say for certain why the GPS device stopped sending location data.

3. We do not know when or where the driver may have stopped for a break.

4. We do not know if Arthas is still alive or if he was able to jump off somewhere.

• That Arthas has survived this ordeal as unharmed as possible.

• That he was able to get off at the vehicle’s first stop.

• That this place was a gas station or rest area in Germany or a nearby country (Czech Republic/Austria).

• That he holds on and manages to be found by kind-hearted people, wherever he may be.

• That someone finds his collar.

• Or some other kind of clue.

• That the Bulgarian driver hears about the story and comes forward.

• That Arthas comes back to us!