Is there an equivalent of proprioception for objects? What is it called?

You don't know that there is a plant behind you. Rather, you suspect that a plant is there because one was there the last time you looked and you don't think anything has acted on the plant to change that condition. If people "knew" what was behind them, there would be no sneaking up on anyone. Proprioception is the brain knowing where in space the body is. This is the sense that allows you to swat at a mosquito on your arm without looking, for example. Some people have disorders of this condition to some extent or another, manifesting itself in ways such as they think they are being followed. For the outside world, the word is exteroception, but that describes the perception of outside stimuli such as hearing a sound or feeling a texture. In the case of the plant behind you, you have not sensed anything but rather, are describing the lack of sensing any changes to the plant's position when you last sensed the plant. "Spatial intelligence" is part of Gardner's theory of multiple intelligences, which has very little mainstream acceptance in comparison to the dominant general intelligence factor. Gardner himself admits that there is very little evidence for his theory. A better term would be "visuospatial ability", which is the ability to perceive spatial relationships.

http://en.wikipedia.org/wiki/Spatial_intelligence is a broader term. Wayne Gretzky is famous for his. "...http://www.thetwosecondadvantage.com/resources/the-two-second-advantage-chapter-one.pdf the whole evolving situation – everything that was happening on the ice and the movement of every player – in his mind. ... He built a predictive model of hockey in his head, so that as a game unfolded he could use memories of past games and tactics, and a reading of the immediate situation, to predict what would happen next. ... “He reads where other people are going to be,” said Grant Fuhr, who played with Gretzky on the Oilers. “People don’t even think of a play, because they don’t think that play is possible. And Gretzky makes that play. He’ll pass to a place, not a player. Somebody’ll be heading to a place, and Wayne knows they can score from that spot, and that’s where (the puck) goes.”

Uh, isn't this just "memory"? I know there is a plant behind me because I saw it earlier and remember that it is there.

Spatial awareness?

http://en.wikipedia.org/wiki/Object_permanence?

The OP remembers the plant, knows it hasn't moved and thus knows it's there. If it's a plant in their private office and they just sat down, then yes, they definitely know it's there. You most certainly can know, by weighing all the possibilities and observations of the area. A plant isn't going to disappear into thin air. I think "know" does not mean what you think it means. It doesn't mean "I think so", "my best guess", or "the last time I looked". Ever been the victim of, or seen someone be the victim of, the cruel stunt where a chair is pulled out from under them right before they sit? All of those people "knew" a chair would be there upon which to rest their buttocks. You are distracted by the plant and the OP's particular hypothetical. The point is that you don't know what is behind you, although you can often have a pretty good idea. Yes, it would be pretty hard for a plant on the shelf behind my desk in my office at work to be moved because I sit facing the door. However, I can fairly easily take a piece of candy from the bowl that sits at the entrance of a secretary's cubicle because she sits with her back to the bowl. Before and after I take the last mini Snickers, she "knew" that there was one mini Snickers left, but she would be wrong. Of course, this is obvious when considered the other way. While I am working at my desk, I would have no idea if a beetle were to walk across the shelf behind me. Absent some spider sense, I cannot know the beetle is there anymore than I can "know" the plant is there. What you are calling "know" is what most people call "remember".

The OP remembers the plant, knows it hasn't moved and thus knows it's there. If it's a plant in their private office and they just sat down, then yes, they definitely know it's there. You have a much more concrete view of things than I do. "Definitely" is too strong a word. "Is pretty sure" or "would bet a decent amount of money" is something I would say. But the point was you don’t know. Not talking about how good the odds are that the plant is still there, but about how it relates to the senses. If you turn around and look at it, or keep one hand on it, then you can say you know.

It's called zanshin in several Japanese martial arts traditions. Usually refers to an awareness of your space and surroundings in case you need to react to them.

Wikipedia calls it the http://en.wikipedia.org/wiki/Sense#Kinesthetic_sense as well as Proprioception when it's for body parts, but I'm not sure they can be applied to outside objects.