This is standard procedure for all professional car thieves.
1- take vehicle
2- Park it someplace that they can observe.
3- Leave for three days, if it is recovered, it has a tracker
4- If it is not recovered, flog the car on, or stick it on the next ferry to the continent.
Unfortunately the thieves knew what they were doing.
Sadly, very likely true.
Immobiliser activated cyanide gas canisters might be a better solution than a tracker?