The DeLorean time machine had the ability to convert into a suitcase which Doc had invented to store the car. The DeLorean would contract and shrink and then finally seal shut only to open at the sound of Doc's voice or Einstein's bark.
The suitcase would be unmovable by normal effort, as it was still a car, but a forklift could pick it up and move it around. This feature could only would stop the DeLorean from getting stolen if it was parked in the street and it made the time machine easier to hide.