If lenders want an early warning that you’ve been fired or demoted, Equifax’s Work Number service can hand one over. “This critical and timely information will maximize your efficiency on credit risk and collections decisions,” the company promises lenders. Armed with information about what you’re making or if you’re no longer working, they can turn up the heat on efforts to get you to pay or reduce credit lines accordingly.

Equifax can also use the payroll data to help colleges track the financial progress of its alumni with its “Graduate Outcome Metrics” offering, allowing schools to avoid expensive surveys and what Equifax refers to in its marketing materials as “self-reporting falsehoods.”

And if your mind wandered to where mine did in imagining other Work Number uses, yes — employers can and do ask for job applicants’ permission to check their current and previous salary where it is legal to do that. So fibbing about your past compensation in hopes of securing a raise may not work out so well.

Speaking of falsehoods, I found something that looked like one on my own Work Number report. (You can get yours free on Equifax’s website the same way you would a normal credit report.) It said that in June 2017, Discover Financial Services was able to dive into my Work Number data.

But unless I’m forgetting some long-ago dalliance, I’ve never had a relationship with that company. So why — and how — has it been able to pry? I figured this was a mistake; credit reports tend to have lots of errors, after all. Equifax suggested disputing the item online through its normal process. Discover was unable to offer up an explanation by my deadline.

In the wake of the payroll unit breach that Mr. Krebs reported on, which resulted from thieves using personal information from affected employees to reset their passwords, the University of Louisville stopped doing business with the Equifax service. Another company, the building material manufacturer Saint-Gobain Corporation, made a different call in the moment and kept Equifax while also starting an examination of competing services.

Erickson Living, a retirement community operator, also continued doing business with Equifax, while adding additional security measures and shutting down online access to W-2s. The aerospace and defense company Northrop Grumman declined to say what it did, and Mark Root, a spokesman, declined to say why.