I looked at my wpa_supplicant.conf. None of the networks have scan_ssid set. I'm unsure how often this happens. I have a probe logger running 24/7 at my apartment with four radios, but it only logs the first probe it sees for a given SSID for each client radio MAC address. The only time related piece of data I have is one instance where I know there were about three days between the first time I connected to a network and when it showed up in my log, but that really doesn't say a whole log.