seen this happen a few times today. once i saw a dot but had no dar bar, paid no attention to it and next thing i know a 190 is shooting me down. 
I would trust a dot long before I trusted a dar bar.
The Dot is supposed to appear at anything over 65 feet AGL.
The Dar Bar is supposed to appear at anything over 500 feet AGL (IIRC).
Which means that aircraft between 65 and 500 feet within radar range will show a dot, but no bar.
I've seen evidence pointing to malfunctioning of the Dar Bar at times, but that's already been discussed a million times over.