Reddit mentions of MMG New Cable Lock Armored Security Locking Steel Cable - 2 Keys - 40" Lenght x 20mm

I have some of these thick steel cable locks. You can hook them together as long as you wish, but 2 will usually work fine for a scooter. Maybe there's something similar in the BENELUX area.

I have purchased motorcycle alarms with 2-way remote like this one. The alarm notifies you on the pager with the sensor that was triggered.

I've also purchased GPS trackers like this one. It uses a GSM sim card to notify you by text message if the tracker has left an area that you define. It can also text you if it travels faster than a defined speed. It doesn't only rely on GPS for tracking. It also uses GSM/GRPS to triangulate a position and give you an actual street address. You can call or text the device to engage constant tracking with updates every few seconds or minutes.

One of my scooters has a remote start with engine kill.

There are also alarms like this one that have a proximity sensor that triggers an audible warning if anyone approaches the scooter, besides sending notification to the remote control pager. I haven't purchased this one.