my question is:
a) how are they downloading all your assets? (do you have it on some unlimited plan?)

I think so. Envato AFAIK doing.

b) curious also - how are you finding your assets/finding out the ones they've taken?

thanks!

Untill I find them taken I don't know about that. (titles/keywords + sorting results by popularity if you meant this)

"that they have bought from a contributor"? What does that mean? What contributor of what agency? Because I do not know any microstock agency that allows re-selling of images, so, yes, that would actually be illegal.
Did you file a DMC copyright notice? They are legally bound to react to that.

Yes I did. Dozen e-mails with DMCA-bla-bla "I copyright owner, this is violation.... this is link to my port at Alamy.... bla-bla"
No in - no out.

But.
Hold tight, guys, I have another sad story linked to this case.

Once upon a time I signed to Ingram Images agency, uploaded 2k+ images and first couple years had 70-100 pounds every quarter.
Then income fallen like I newer saw before - immediately and was like 20-30 pounds/quarter.
Couple years ago I wrote letter to Ingram with request of agreement termination and then-editor-some-guy Louis assured me that all images are deleted, money paid, we all set.
....
Everything was fine untill I decided to sign up to Freepik and upload some of my early "christmas" images there. Next day I got copyright warning from Freepik and demanded to explain where from I got mine (!) images.

I started searching on Freepik markretplace and found.... ALL my "deleted" by Ingram port in portfolio of user....... wait for it.... INGRAM!)
================
Today I asked Ingram second time to do the same - delete my images and terminate agreement.
They agreed.
Again.

Tank you. 
Yep. Many sales and number is growing. I wrote to [email protected], but something (sales🤑?) is preventing them of strong action.
Whatever, will write again.
Found in that port photos of another two authors,  but cant reach em.

Happy New Year!
(new user)