While the official MicroPython has "wlan.status('rssi')" to to retrieve the RSSI of the AP signal (when you are in STA mode connect to the AP), Pycom firmware >= 1.20.0.rc0](https://forum.pycom.io/topic/4099/new-firmware-release-candidate-v1-20-0) has a new function :

wlan.joined_ap_info()
Returns a tuple with (bssid, ssid, primary channel, rssi, Authorization method, wifi standard used) of the connected AP in case of STA mode.

Tested it with firmware 1.20.0.rc4 on LoPy4 and it works.
So no need to call 'wlan.scan()' when there is already a connection to an access point.

Also note that the same firmware 1.20.0.rc0 brings new features (for example, to filter by ssid or bssid) to 'wlan.scan' :

wlan.scan([ssid=NULL, bssid=NULL, channel=0, show_hidden=False, type=WLAN.SCAN_ACTIVE, scantime=120ms])"