They are a murder investigation unit so they were called in for the CHIS - someone was already dusting the balcony for prints so obviously they were already underway in their process.
Ah yes I was forgetting they were a general murder squad. Though it was still rather convenient they put 2 + 2 together so fast and determined the random dead person was the CHIS.
Steve only paid attention to that bit of the CCTV because he was trying to establish whether she could actually have seen something suspicious - that’s what he was looking at, not the crime itself. Makes sense to me.
I still think he should have been equally paying attention to the crime as if the whole thing was a diversionary tactic there could have been signs it was a set up. The timing had to be very specific for it to work.