What does the word 'nimitta' ('sign') refer to in SN 47.8?
In the same sutta, buddha said:


  tathā hi so, bhikkhave, bālo abyatto akusalo bhikkhu sakassa
  cittassa nimittaṃ na uggaṇhātiฯ.


Then said about satipaṭṭhāna:


  evameva kho, bhikkhave, idhekacco paṇḍito byatto kusalo bhikkhu 
  kāye (=base, object) kāyā-nupassī (=sati-cetasika) viharati ātāpī 
  sampajāno satimā, vineyya loke abhijjhādomanassaṃฯ


So:

cittassa nimittaṃ (base[=object] of consciousness) = satiya paṭṭhānaṃ (base [=object]  of sati).

The other name is kammaṭṭhāna (base[=object] of cetanā).

P.S. Sati must arise with consciousness.

I'm sorry for my first mistake answer.