In Back to the Future Part II, when the old Biff steals the DeLorean in 2015, Marty is just a few yards away. He might not have seen it because he was looking at his future house, but he must have heard it. The street was empty and quiet. Clearly it was not possible not to hear the DeLorean engine starting. And yet he didn't even notice anything.
The official novelisation offers a brief explanation. In short, our trio of heroes were simply too pre-occupied to notice the DeLorean being taken.
On top of that, with the hover-conversion the car is also much quieter than a normal petrol-driven engine. Additionally, the car was only gone (subjectively) for a few seconds.
And all three of them. Doc. Marty and Jennifer, were busy elsewhere, so none of them could see Biff take off in the DeLorean, or, a moment later, land the DeLorean in the exact same spot. He quickly got out to hobble away, too quickly, maybe, because his cane got caught as the gull-wing door swung down. The cane snapped in two as Biff struggled with it. He took the half he had freed and hobbled away, leaving everything like it was before - except that now Biff no longer clutched the silver bag.
We can also suppose that any sounds would have been masked by the hover-taxi leaving the area.
He [Biff] walked straight to the car, opened the door, and climbed inside.
What was the old guy doing? Was that his car? Fred decided he didn’t want to know. He turned the cab around and headed out of Hilldale.
The film script indicates that he merely waited until the "coast was clear" and Marty wasn't in the immediate vicinity.
EXT. AT THE DELOREAN
MARTY, now back in his 1985 clothes, impatiently looks in the direction Doc went. Should he take a closer look or not?
Yeah. He quietly follows, leaving the DeLorean unattended and out of his view.
Immediately, BIFF, clutching the silver bag, comes up to the DeLorean, sees that the coast is clear and opens the driver's side door.