I'm still on Mavericks 10.9.5 and today a random device popped on Finder. I live in a dorm and we use the university network, but I'm connected to the wireless network of my TP-Link TL-WR710N. That got me concerned for security (whether someone was using my network or not). My router is configured to disable SSID broadcast and uses WPA2 with a 63-letter password (a random mix of uppercase, lowercase and some symbols like /\!()$%&). MAC filtering is enabled and only three devices are allowed: my Macbook Pro, my Galaxy S4 and my Kindle (no girlfriend, unfortunately), and I enable friends' devices when I need, but then disable them again. I use Chrome with Flash disabled. However, firewall and file vault were not activated until today. When this device popped up I had bluetooth turned on for the mouse and the Console showed some AirPlay messages. Could this be a device of an intruder? What further steps could I take to improve security? How do you usually secure your Macs/networks? I appreciate any advice on this.