Some parents never make the adjustment to the fact that their child is now an independent adult.
The adjustment is usually reinforced when the child stops living at home, and parents accept that their child is now an adult with the same rights to privacy as they have.
It seems in this case, that your parents have not made this adjustment, and to an extent, you are facilitating that situation, even though it does not suit you.
Standing up to your parents and making them see that you are now an adult can take a little time, and be uncomfortable, but it must be done.
Ask for the return of your keys, and be honest about why you want them. Going into your house when you are not there is disrespectful, but fits with their perception of you as still being their little boy.
Bite the bullet and sort this out, it will be better for everyone.