It's curious isn't it? You're cautioned to be vigilant, find a cancer, hit with massive amounts of science in whatever your treatments are, and then it's 'cheery bye, come and see us if there's a problem' as the door slams shut behind you! As @lrb_03 says statistically it's been proven there's no benefit to extra scans just because you've had cancer.
My breast surgeon and oncologist will physically examine me every few months, decreasing as time goes by, and that's it. No ultrasound or anything.
It took me a while to get my head round it. I've had a double mastectomy and reconstruction. I've reduced my risk factors massively, done everything I could. It could recur in my scar, my skin, my armpit, the tiny bit of breast tissue that remains in the fold of skin beneath my foobs and in my chest wall, to which my tumour was very close. It's the latter that bugs me. There's just no way of me ever knowing if there's anything going on in there. I just have to cross my fingers and hope for the best. K xox