Well the answer depends on the type of dive and the dive site. For shallow dives (less than 16m) at our normal dive sites, the captain follows bubbles and if he loses them, he just circles the sites waiting for the smb to pop up. This works well here with the dive sites we are on daily.
Now for deeper dives, the captain knows we will be shooting an smb once we hit 21m. Once we descend, he begins looking for an SMB right away, Either a yellow one for an emergency or an orange one indicating we have begun our ascent. So no matter if we begin to ascend early, there is an SMB being shot that he can find and then follow us even if we are doing a drifting deco.
As far as location on the deeper sites, the captain knows where we are heading. He is also very aware of the currents that we may encounter and where those would send us if we get caught. So with this info he is constantly scanning in the known possible directions, again looking for the SMBs.
This process is one that has served us well due to the specifics of the dive sites, the knowledge of the captains and the communication between dive team and captain.