'di sini' means 'here' e.g. Kamu sudah lama di sini? Have you been here for a long time? or Dia tinggal di sini. He lives here. Note, di sini can also be written as one word disini.
Disini can't be accepted as one word. Although many Indonesian peoples write disini, it's wrong. Di sini: here/at here/in here. Dari sini: from here. Ke sini: to here/come here.
I’m not 100% on this, but think it’s just being shortened when they use sini. It’s seems to be fairly common for them to shorten words in Indonesia. But I haven’t been learning this all that long yet
This is actually very common in spoken Indonesian.
"Kamu dari mana?"
"Dari sini!"
I would always say that if I was tired of people asking me where I was coming from!