Yes, Proof and sets only in 2011. I doubt that yours is the proof version although not entirely impossible. Unfortunately, it is not uncommon for people to break up coin sets for reasons only known to them. We can only speculate. I would still hold onto it as there were so few made and I am sure some collector would want it unless totally trashed which from what I see on your photo does not seem to be the case.
Then again, the frequency on the coin page is slightly higher than the other years “only in sets” and most importantly, you seem not the first one to find it in circulation as there are a few for exchange in AU or without mentioning it being in a set. So there is always a possibility that there were some extras released into the wild.
Would be interesting to figure out if there are more people owning this coin in lower grades than UNC. Also the 10 cent of that year seems to be less common N#9895 (also only in sets) than this 50 cent.
Just call me Bram
Oh! And do read my profile page before you open a swap ...
I have the experience that many coins in sets would hardly pass as UNC with all the scratches and dings they come with straight out the mint. Even so called “BU” sets I own contain coins that look like they were just dumped in a big bucket with all the others before being entombed in plastic.