Yep, looks like you're missing the bushing. It appears from the picture that the bolt head snapped, because to me it looks like the bolt shank is still protruding from the nut.
Would they need to remove it for a spacer lift install? Not really... But they might have to make things a little easier. Maybe they snapped it off when working on it and left it. I don't think the bushing could really just fall off by itself. It clearly hasn't been like that for a long time when you look at the dirt on the other parts.
Have you noticed it driving more squirrelly or any additional body roll?
Hope that helps.. Good luck and definitely get that fixed.