Under cnMaestro we can see statistics for currently connected "Wireless Clients", specifically MAC, WLAN, Band, Upload/Download etc.
From what I can tell a client is not connected you lose all data and visibility of that user, and thus any way to check what that wireless client's last connection statistics were.
I would propose retaining this information for a set period, perhaps under an "offline" clients section or toggle.
Thisway we could refer back to see what wireless clients have used data wise, their signal levels, etc which I feel would be very useful.