Antonym of "target"
Solution 1:
Noun
nontarget (plural nontargets)
- That which is not a target. Wiktionary
First subjects were trained to search for occurrence of any one of five target words among a list of semantically unrelated nontargets. Theoretical Models and Processes of Reading, Vol. 1 (1994) p.874
As wind speed increases, the impingement force of spray droplets will increase on targets and nontargets. This situation creates considerable variation in control efficacy and can often influence whether the effects on nontargets are minimal or substantial. Encyclopedia of Environmental Management (2012) p.1472
These new data will be used for risk assessment of nontargets to treated baits and snake carcasses containing the acetaminophen. Research Update (2001) p.8
Although I found an example of the adjective nontarget dating back to a 1832 SCOTUS document (nontarget species), definitions of the word as an adjective (more common) or noun seem to get short shrift.
Solution 2:
For things like poisonous mushrooms, or other undesirable items amongst a mix of desirable and undesirable items, an appropriate word could be bogeys.
In terms of physically shooting at a target, the non-target area could be considered the wild or the rough.
Also, usually a "target" is conceived as a relatively small area of a much larger environment. If the general situation is the opposite, where the desirable area is large with relatively small areas to avoid, you'd might call the small areas traps.
The word target has many senses, so there is no single antonym that covers them all.