Shodan Monitor will do it and if you're only keeping track of <= 16 IPs then you would just need the membership which is a one-time payment of $49 (i.e. no subscription cost) for a lifetime account upgrade (https://www.shodan.io/store/member). You just provide an IP/ network/ domain and we'll notify you if anything changes or becomes vulnerable. It's basically Google Alerts but for network ports:
Disclaimer: I'm the founder of Shodan.