A major service provider in Zambia has lost thousands, possibly millions of USD after they left UDP through port 53 uncharged...completely for about 1 year .People quickly discovered that if they used a proxy like cproxy or 'Your Freedom ' ,they could browse and download for free using 3G service.
Ofcourse the slightly more technical fellows using a VPS and openvpn udp proto , could achieve the same. One group on facebook of people helping each other set this up ,had at last check 3600 members, Impossible to know numbers of non-facebook users and with internet bundles charged at average $40 per 2GB ,add that to young people freshly introduced to torrents on basically unmetered internet and you have some huge usage.
They finally silently plugged the hole last week leaving most people in the 'internet dark ',and looking for the next hole, one thing the network has on their side is the reletive low level computer know how of these fellows so most 'probing' involves changing the port or IP of the external proxy.I do not think anyone is running stuff like nmap or investigating other IP over x tunnels.
They are not alone though, last year another ISP lost again possibly millions due to a bug in how the calculated bonus points, which allowed a person to buy pre paid airtime for $2 ,use that to buy a 10 MB bundle and after using it,you are awarded 10,000 points which you could then change for a 2GB bundle ,hours of talktime, thousands of messages or silly physical materials.That hole was plugged 3 months ago.