wireless networking - Is it possible to hide one's presence from the DHCP client list

I ask because I recently changed my WiFi password after seeing some unfamiliar devices in the DHCP client list.

After that, the client list only has familiar names now, but there is still some inexplicable behavior in the network (network getting congested when it shouldn't. WiFi router blinking some lights that shouldn't be blinking (didn't before), etc).

These problems MAY be coming from other sources, but I just want to make sure that it is NOT possible for an intruder to hide himself/herself from the DHCP client list, before moving on to other issues.