Indeed, happened to my very privacy sensitive colleague while setting up his new S3 (he had no prior G+ account, so he got a new one). This may depend on what type of account you have and what terms you have agreed to.
How did you confirm that? Google+ on my Galaxy S III asked me whether I wanted to allow it to upload pictures on first launch (not surprising as it runs the same G+ app as every other device). Of course most people will simply do what nnnnni stated, which is a "sure whatever" clickpast (which Google knows and takes advantage of), and forever more declare it unwanted, mysterious behavior.
1. buy new note2 from att, register google account, skip samsung and att setup.
2. buy new s3 from amazon unlocked, register google account, skip samsung setup.
3. never even open G+ app on both phones
4. take a picture
5. wait an hour
6. you get a notification "pictures you took are ready for sharing" i.e. they are already uploaded against your will and out of your knowledge.