I agree with several of the comments above. It is probably a static address assigned to a printer or other device, or an improperly configured second DHCP server.
If you have devices that cannot DHCP for an address, you should assign them a static address but reserve the address in DHCP....that way you can clearly see where all of your IPs are assigned. Mind you this has no technical benefit other than it makes sure the DHCP server doesn't give out the statically assigned IP address to a DHCP client.
If you have 2 DHCP servers servicing the same subnet, you MUST make identical reservations on both servers...also you MUST EXCLUDE IP ADDRESSES for the other DHCP server. DHCP servers do not communicate their address pools to each other so you must exclude half of the available range on DHCP server #1 and the OTHER half on DHCP server #2. This prevents the 2 DHCP servers from giving out the same IP address to different clients. The reservations can be served by either server because they give the same IP to the same client...in fact you MUST reserve IPs on both DHCP servers in order to be assured the reserved client always gets the assigned IP.
