I would like to add my voice here to those asking for the ability to set and an alert for unusually high activity, but not for unusual low. Here's an example of why this is important:
Recently we were victims of a DDOS attack. We got plenty of alerts when systems stopped responding, but we could have been spared a lot of grief if we had spotted the increase in network traffic first.
No problem, I said, I'll just add some PRTG traffic sensors to our outward-facing web servers, and get alerts when traffic is unusually high. That would be great, but in order to do so, we'll also get alerts whenever traffic is unusually low -- for example, every time one of our web servers switches to another host in its pool, and traffic drops to zero. This is perfectly normal behavior, and we don't need to be alerted every time it happens. Worse, if we turn this feature on, we'll get enough false alerts that people will start ignoring them.
I've searched the Knowledgebase and found many requests for this feature. I can't think of a single instance when I would need to know when traffic is low (if it's a problem other sensors will trigger), but many cases where I would like to know when it's high. The "unusual" trigger is a potentially fantastic feature of PRTG, and one that as far as I know, none of your competitors offer. But as it is , without being able to turn off the unusual low alerts, we cannot use it.