Wi-fi light on router

[#1]

I've recently set up my 582 router and there's a button/light on the top for wi-fi.
I've been into the router settings and enabled power reduction mode, which says that when wi-fi is not being used, this function on the router is de-activated. Thing is, I haven't connected to the wi-fi for over 12 hours and yet the wi-fi button/green light still comes on intermittently.  Nobody is connected (it's not the flash that occurs when the wi-fi is being used) - the wifi light just comes on for 30 seconds or so at a time at regular intervals.
Anyone else noticed this? The wifi button doesn't get referenced in the user guide (or the WPS button for that matter)

[#2]

So, when the wifi is deactivated as it is not being used, how does a client connect Presumably, the wifi is turned on for a short period so that a connection can be made and power raised for the session

[#3]

I see what you're saying, but if I disable the power save mode then I think the wi-fi light exhibits the same characteristics, which makes me wonder what the power reduction mode actually does.
I know with other wireless routers, the wifi light only comes on when you attempt to make a connection to it. I can't see why the light would preiodically come on on the 582.