No it's not downloading huge files that triggers an alarm. I have an unlimited plan and I can stream TV unlimited on my phone without issues. But if I tether my phone I will get a nastygram text within 48 hours.
I tried PDANET. I got the same nastygram 48 hours later that I did without it.
Then you didn't have the hide tethering on or at least working properly. When I have it off my tether data goes up. When I have it on my regular data goes up. Sorry you had issues with it. But when working properly they can't tell you're tethered.
Last edited: Jun 11, 2020
Reason for edit: Added screenshot