Do you know who's currently connected to your network?  Are you sure it's just your PC and your router?  What about all those other IP-capable devices in your house?  And what about all those other IP-capable devices in houses in your street or your block?  Are they connecting too?  

Your router can probably give you such information, but it requires logging into the router each time and the list doesn't update automatically.  Which are just 2 of the reasons why I rather like Wireless Network Watcher, from Nir Sofer's excellent "nirsoft" web site.

Despite the name, Wireless Network Watcher will watch any small network that consists of both wired and wireless clients.  In the example shown below, it's picking up routers, IP cameras, smartphones and more.  You can show all sorts of useful information columns, such as the MAC address of all devices and the date they were first detected.  Plus, as mentioned, the display updates automatically, so keep it open on your desktop to watch users and devices come and go.

The easiest way to try WNW is to download from http://www.nirsoft.net/utils/wireless_network_watcher.html (click on the Download In Zip File link from about two thirds of the way down the page).  This is a portable version that's only around 0.3 MB to download, needs no installation, and runs on all recent versions of Windows.  The program is malware-free according to VirusTotal and Web of Trust.  If you don't find it useful, just delete the unzipped folder and you're done.