Can an Auto roll back in D

Told you don't be afraid to ask what you think are silly questions.  Keep asking questions, as that is the only I learn things!

Yes, an auto will roll back if in D on a steep enough grade and in the case of multistory car parks the grade could well be steep enough.

I had 2.0i auto Vauxhall Carlton which would hold my dads caravan on a gradient in D with no problem but my 3.0i 24V Vauxhall Senator used to roll back on hills all the time.  The Sennie, at idle rpm (750 rpm) did not generate sufficient torque to hold 2 tons of machine still on the hill.  Once the revs were about 900 rpm she would 'creep' up the hill as the torque from the engine was sufficient to overcome gravity.

Get the tranny serviced just to be safe though!

Andrew