Liens are legal matters that are granted by the courts. If this is legitimate there will be some record of it on file with the state. Contact your county clerk's office and see if there were any judgments levied against you that you were failed to serve papers for.
But this strikes me as an extension of the invoice scam-- send an invoice for fake goods/services to random companies and hope some clueless AP clerk cuts you a check. If they couldn't find you to serve papers at your home or workplace, how was it so easy to send you a lien letter?
I bet the lien amount is for less than $10k as well-- the magic number below which scammers are immune to lawsuits and prosecution.
That this lienholder claims to be working with your mortgage lender strikes me as even more suspicious. Liens impose a pecking order in terms of who gets paid first in the event you become insolvent. Banks are not in the business of charity and sharing-- they want their money from you, first, and to hell with everyone else. Nevermind working together, these two entities should not have any reason to be talking to each other in the first place. It's easy to pretend to be though given how easily your lender's name can be divined from public documents.
Do not contact this supposed creditor, at all. Not even to disavow the debt. If it's a scam, opening a line of communication is just playing into their game. Besides, according to them, they already won by virtue of being granted a lien. They get paid when you sell the house. If they're demanding cash now, they're trying to con you.
If you feel the need to discuss this with anybody, find a bankruptcy lawyer (they handle debt-related issues like this) and have them verify legitimacy. They'll know exactly who to call or what databases to look in. Many offer free consultations, and if this turns out to be fraudulent, they may even be able to shake this company down for FDCPA violation fees and earn you some money.