Try accessing a site such as http://www.whatismyip.com/ via the wifi router, to see if the external IP of the hotel router matches the one you've found connecting to your mail server. In an attempt to confirm whether its likely the laptop is in the hotel.
You could also try a tool that maps an IP to location e.g. http://www.geoiptool.com/ - these aren't particularly accurate too be honest
Edit: If you know the Laptop is running a server e.g. SSH
you could try scanning the network for the local IP of the laptop with nmap.
e.g.
nmap -A 192.168.1.*
(With the '-A' option, machines will also attempt to be fingerprinted, to determine their OS etc, to start with it may be better running without '-A' as it takes a fair bit longer)
Comments
Try accessing a site such as http://www.whatismyip.com/ via the wifi router, to see if the external IP of the hotel router matches the one you've found connecting to your mail server. In an attempt to confirm whether its likely the laptop is in the hotel.
You could also try a tool that maps an IP to location e.g. http://www.geoiptool.com/ - these aren't particularly accurate too be honest
Edit: If you know the Laptop is running a server e.g. SSH you could try scanning the network for the local IP of the laptop with nmap.
e.g. nmap -A 192.168.1.*
(With the '-A' option, machines will also attempt to be fingerprinted, to determine their OS etc, to start with it may be better running without '-A' as it takes a fair bit longer)