It turned out to be totally legitimate.
After confirming that the person who IM'd me did in fact work for the company I gave her my email. She passed it on to the benefits administrator who contacted me by e-mail. It turns out that when they'd calculated my severance pension amount, they'd under calculated one portion of it, and she sent me the pension statement showing the calculation and balance due.
It's not a huge amount of money - just under 600, but heck I'll take it. It'll help me pay off some debt.
Now I just have to have a phone conversation with her, this morning, to figure out the logistics of getting me paid.
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"I danced in the morning when the world was begun. I danced in the moon and the stars and the sun". From my favourite hymn.
"If you see the wonder in a fairy tale, you can take the future even if you fail." Abba