Okay that's definitely interesting!
So given the current switch @terminal3 listed how do you tie that in with say an EcoLink sensor? It doesn't look like to me you can directly connect the switch terminals and the available internal Ecolink terminals without frying the sensor.
Since this is not really my area of expertise I am probably missing something..
That is exactly what you do. But there is no need to put any external voltage at those terminals - the wiring diagram for that sensor is for a different setup that is closer to what I do for my in-duct blower.
When current is detected, the NO switch closes, registering as a "closed" event on the Ecolink (or GoControl). When there's no current, the NO switch returns to being open, and the contact opens.
So ARGHHHI missed the part in the diagram where you need to supply power and a load to the secondary.. because of course you do 
I thought it was "induced" by the primary for some reason like a transformer, sigh... If you don't supply power then no worries. It does mention this bit though:
- NOTE: If you want to test the operation of the sensor switch, a load must be applied across the NO contacts; otherwise, it will not work.
Looking at the stuff you use - that looks even more flexible as you don't have to remove any wires, just clamp on.
Yup, the split core CTs are really convenient. The only thing to keep in mind is that to only pass the load conductor through the CT. You can increase sensitivity (induced magnetic field) by looping the conductor a few times through the CT.
These "bacon sensors" are extremely sensitive and will probably trigger from vibration on the frame when the fan is running.
Aqara Vibration Sensor, REQUIRES AQARA HUB, Zigbee Connection, Wireless Mini Glass Break Detector for Alarm System and Smart Home Automation, Compatible with Apple HomeKit, Works With IFTTT Amazon.com
Another potentially simple option would be if the fans have duct work that has louver that open when the fan is on to allow it to run simply attach a door sensor to them.
Fans kind of vibrate while they are going, is there anything (battery controlled?) that can detect a kind of humming vibration? To eliminate false incidental bangs and clatters you can use "and stays that way" in a rule.
Or maybe a little camera or microphone so you can see what's going on by looking and listening. Thats what we ended up having to use to check if our dehumidifier needed emptying.
That's good too.. but I kinda like the idea of detecting the current in the wiring. Seems like less potential for false positives.
With a split current switch like @aaiyar uses it seems relatively trivial to install - and given the EcoLink has built in terminals internally no soldering or anything required.
You would have to buy 2 devices though... and I would probably hardwire the Ecolink so I would not have to replace the batteries.
Yup, it is a direct test of whether the fan is drawing power or not. And while the initial outlay does require two devices, as you state, the reliability and accuracy are excellent. Never had a miss!
Exactly what I do for this application - my HVAC/Sonos GoControl contact sensor is hard wired. It is basically underneath the air-handler, and hardwiring it ensured I wouldn't have to contort my (aging) back to replace batteries 
In my experience, they were insufficient to detect when my dryer was running. Here's what would happen:
So when the dryer actually ended, there was no event to indicated it had ended. Others got around that by mounting the sensor on a coil spring and attaching it to the dryer or washer.
I would expect if you can do it both would probably be good. As much as you want to know if the fans are running: you probably also would like to know if you have a direct opening to the outside when you shouldn't.
Out of curiosity what is controlling the fans now. You indicated they are controled by humidity and temperature so they do have some smarts to them.
Are they home exhast fans, or simply for atic circulation of air? Is the humidity reading from with your home, simply about your attic, or does it do some kind of check of inside and outside? If they are exhast fans do you try to open the windows when it is running to help it cycle air through the house better?
Try something like this:
They are hardwired roof fans that blow air out of an insulated attic. The roof also has a few vents to allow air to enter attic when fans turn on. Each fan has an integrated temperature and humidity sensor, and I can set what temperature or humidity they should turn on.
All you guys have come up will some great ideas. Iām really liking the Current Relay and Contact Sensor idea! Iām not much of a hardware guy, so tell me if I understand this.
I run a wire from each of the two terminals on the Current Sensor into a battery powered Ecolink contact sensor. No other power source required. Run the hot wire going to the fan through the Current Sensor loop. Pair the contact sensor to Hubitat and bingo, it will work?
Correct. I would suggest making sure you get a CT that can detect the amps in the range drawn by your fan. There are ones available that can do 0.1-100A that would be perfect.
Also, I really like current sensors made by Functional Devices here in the US. They just last forever. This split core sensor will be perfect:
This is my favorite in the "cheap and easy" category. Nicely thought of.
However, monitoring the current seems like the way to go as it seems 100% reliable and since the item being monitored is out of sight, being sure of the status is important.
And as a big plus, @aaiyar offers one year of free phone support! 
@daleberlin - I would be careful about using battery powered devices in your attic especially if it gets really hot. Consider hard-wiring the battery terminals.
@daleberlin - this is a good idea. But why don't you get it working first, and then you can redo the battery on the Ecolink sensor.
Also, you can pass the hot or the neutral through the current transducer. Just not both.
Just noticed this and realized I needed to ask a question.
I have a couple leak sensors in the attic, one is a SmartThings and the other is Tuya.
It can get as hot as 100Ā° here occasionally in the summer these days, that's still pretty rare but we do see a fair number of days in the mid-90s.
Sounds like you're saying that having batteries in the attic in that kind of heat is not a good idea.
So ymmv of course but in general the hotter the less optimal the battery situation.
Here's some jibber jabber:
https://www.sciencedirect.com/science/article/pii/S1002007118307536
tldr:
- Conclusions
Thermal challenges exist in the applications of LIBs due to the temperature-dependent performance. The optimal operating temperature range of LIBs is generally limited to 15ā35 Ā°C. Both low temperature and high temperature out of this scope will affect the performance and may cause irreversible change to the LIBs.
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