ISP scares the Hell out of me. I only use it because I've moved in with family in the past month while house sells. Have mining rigs set up. Never had any problems with TWC, but Comcast seems determined to monitor EVERYTHING that goes through them.
About a month ago, Comcast insisted there was malware installed on computers in this house (family member tends to have malware on her computer, but was odd that this first came just after I set the rigs up). I checked all the computers -- nothing unusual going on, no concerning network traffic, no p2p-software (outside Bitcoin) was/is running. This morning, Comcast sent another email saying they were blocking port 25 due to "detected virus-like activity from your modem." Checked, and there's no network traffic using :25. Is this all due to Bitcoin mining traffic? Anyone have similar experiences? Becoming concerned they're going to try imposing fees or canceling service.
It's possible that your computer is infected by malware that sends out spam. As mentioned in this thread, port 25 is used for sending e-mail. Even if you check with anti-virus programs, there's a small possibility that the malware in question goes under the radar. Also, you'd had to constantly monitor that port to ensure there's no activity on it. For all you know, the activity may happen when you're not acitvely using your computer. Another possibility is that Comcast somehow have target you in error, this may happen as well. Anyway, if you get port 25 blocked, unless you need it to send e-mail (perhaps you could use another port, or another service), you should be fine. Bitcoin doesn't use port 25.
Another possibility is that your miner is infected with malware, if you run binary version you downloaded from the web, you really don't know what's inside that binary, but if you download from a 'trusted' source, you should generally be fine.
In summary, there could be many reasons for this happening, and don't freak out in regards to the bitcoin mining, I don't think this is what they're targetting here.
If you wanted to monitor all network traffic, you must set up a program that can monitor all ports around the clock and which programs are causing the traffic.
Perhaps you could call their tech department, and tell them that you've received their notification, but you couldn't find any suspicious activity on your pc. Then they could (if they want) tell you what they're detecting on their side. No need to mention the bitcoin mining to them at all if calling in, I'm pretty sure that's not the culprint here.