Why did I see many airport and time capsule devices in iPhones AirPort Utility?

I had this problem with my Airport utility that I saw three devices(an AirPort Express and Time Capsule and my own AiPort Express) in my airport utility for iPhone. Were they connected to my wifi network which was created by my own AirPort Express? There were two devices that I think shouldn't have been there. I changed my AirPort Express from bridge mode to DHCP and NAT. Those two other devices disappeared. Could other people somehow have used my wifi network and extended it? It was password protected.