How can I detect Wi-Fi login attempts? I was thinking to put an Wi-Fi interface in monitor mode and look for leaked SSIDs.
To monitor network devices there's essentially two possible protocols: Syslog and SNMP. Both work including for Wi-Fi. But for any detection to happen, you must configure your device to do so.
Many wireless routers log by default failed login attempts. If your actual router device is not sufficient to accomplish this because it does not support certain features, you can upgrade its firmware to DDWRT or OpenWRT.
Once you have a syslog or snmp working, you then can use quite many tools (SNMP Trap Watcher, Kiwi Log Viewer, SysAid Monitoring, SNMP Log Even sentry, etc) to analyze what is happening.