I have recently started using my own sledgehammer-subtle approach of detecting (what I refer to as) Uninvited Activity on any port not offering a service, and straight-up banning the source IP (indefinitely at the moment) from accessing any actual service ports.
Over the few months I've had it running I've needed to progressively create failsafes for IP addresses that I know are trustworthy so I don't lock myself out. I've also started tiering the importance of blocking based on different sets of ports which are being probed. I've also discovered that there's a significant amount of Uninvited Activity coming from "security" companies in their pro-active scanning of the entire IPv4 space - which I don't trust at all and ban with prejudice.
(I'm aware of various limitations and footguns inherent in this un-subtle approach but, as another commenter elsewhere alluded to, "it makes me feel better". I also think that a fair bit of processing volume can be taken off IDS' if a heap of "known garbage" traffic is blocked prior - it's all about tiers).
