IF this was a terror attack then wouldn't this seem like a short-term/spontaneous action? Unless they/he/she observed and planned where that truck would stop when exactly etc
Acts of terrorism organised by individuals or small groups can be very amateurish. This is why lone wolf terrorism can be very hard to predict/stop. When there is cells or group chatter that is when things can be intercepted. Or if people are on watch lists.
If it's individuals or a few who have been radicalized whilst being born in/living within a country there's not always a paper trail to follow them on. Germany is also the country that has potentially let in the largest amount of refugees presumably without vetting the majority of them, or not doing so properly. So even in situations where there could be a paper trail or suspects to follow, they might not have been receiving the surveillance or rejection from attempting to enter the country they otherwise should have if borders weren't so carelessly "open".
Either way, this is speculation on causes of terrorism in a situation where it isn't confirmed to be such a cause yet.
edit: Just realized I slightly mis-read your post. Either way my points stand about why it wouldn't be easy to track someone who in their own time just scopes out where trucks stop and jacks one. Lone wolf attackers like this most likely keep to themselves before whenever they wish to attack and act out an atrocity.
