On FreeBSD we also use the ifconfig command to configure wireless interfaces. The command
ifconfig wlan0 list scan lists available AP along with their SNR. However the ifconfig command does not display the RSSI/SNR on the current link. To do so we need another command, wlanstats. It is not included in base system but you can find it in the source tree under
/usr/src/tools/tools/net80211/wlanstats. This command also shows you the signal level (in dBm) and current transmit rate among other things.
I wonder why the RSSI is not visible with the ifconfig command. I suppose they wanted to regroup all those stats with another command. But again I wonder why this command is not part of the base system.