Actually, it depends on the details.
From the rules: 'If you can honestly say to yourself,
- “Yes, I mainly used Blender to make this.”
- “Yes, nearly all of the foreground models are new.”
- “Yes, most of the models are new.”
then the entry is Pure.’
So, if the object is just a secondary background model, it probably doesn’t matter. If it is an important part of the image or saved the artist a lot of time, I’d go with ‘open’. In any case: it is a good habit to add details like this to the entry post. This way, everyone can make up their minds. Just try to keep things fair. 
Generally: There is nothing wrong with entering open entries. Many great entries have been open in the past. (And many of these have won the challenge.) In the end, it doesn’t really matter all that much.