The parcels are very to notice sitting there, it's shielded with lots of bushes and building on either side.
The thieves must have driven by before, or are following the delivery truck around and scooping them up later. They have some prior knowledge of those parcels being there.
There are tracking devices based upon WiFi or Bluetooth (cell phone, tablet, in-car nav etc). I believe it's constant polling/enumeration of unique device ID's.
Analytics companies make these, municipalities use them "for traffic patterns" along the freeways, and you can also track people's movement in shopping malls for retail analytics.
It's certainly creepy and the thieves would be broadcasting some unique ID's in proximity, if only it was logged.
"Smartphones with WiFi can now be used as an indicator of customer presence thanks to a WiFi mechanism that is common across all such devices: probe requests. These 802.11 management frames are transmitted at regular intervals from WiFi devices. The frames contain information that can be used to identify presence, time spent, and repeat visits within range of a WiFi access point. These devices can be detected by WiFi access points irrespective of its WiFi association state meaning that even if a user does not connect his or her device to the wireless network, the device's presence can still be detected while the device is within range of the network and the device's WiFi antenna is turned on."
Cisco Meraki Location Analytics