OK...I have never heard of that either but old food DOES get pitched in this house -- we *try* not to do that but on occassion you get that one can that somehow gets shoved to the back of the cupboard only to be discovered years later. HOWEVER....I just end up pitching those -- I
hope you didn't OPEN them did you to find the dusty powder? I know you said you shook it, so I'm hoping that is all it was. I would be afraid to open a canned good several years later (unless of course, the expiration stamp said it was good! 
Then I'd still be iffy and more than likely toss it without a second thought!) I know they didn't start stamping those until what I think of is fairly recently but it could be several years by now -- I have been known to say "that wasn't too long ago" to which DH tells me that whatever X is has been around for at least 10 years but it's one of those "but I remember BEFORE they were a common thing".

No, we did not open them! I had visions of what happened when they opened King Tut's tomb and those ancient spores (or whatever) wound up killing them after they breathed them in.

Who knows WHAT substance lurked inside those cans?
MIL grew up with plenty of money, so that's not her problem. In fact, one of the weirdest things about her is that she will assign "value" to a piece of crap and yet treat something that is truly valuable like......crap.

We too can never just pick up something and toss it. We must check everything because that scrap of paper could be a vital legal document or a stock certificate, etc. Or it could be a 15 year old expired coupon for a product she'd never have used.
Not long ago, DH found an expired check for something like $17,000 that she had never cashed. She had just tossed it in the floor with all the other junk and forgotten about it after she couldn't find it. (She'd never ask us to hunt for it because she'd have to endure our shaking heads.) I told DH that if someone in my family lost a check for
$17, we'd be tearing the house up to find it, let alone
$17,000!

When we cleaned out her first house, it was like an archeological dig. As we cleared out the trash inch by inch, we went further and further into the past.

When she moved out of her last house (lived in it for 8 years or so) she told DD she could have all the change she could pick up off the floor. DD collected several hundred dollars.

Not a bad deal. I kept all the Euros and British pounds.

She has valuable jewelry that we have never found. I think it likely got thrown away because it was lost in a pile of junk. She's ruined dozens of earrings by crushing them into the floor when she stepped on them.
She's in the hospital now and we've found hundreds of dollars squirreled away in weird places. I have no idea how much more we'll find. And we have to check
everything. I have actually stopped collecting because her CRAP made me scared for DD. You see, DD is the only grandchild and really, only great-grandchild. Every antique, family heirloom, piece of china, silver, crystal, collectible, etc. from my generation, MIL's generation and MIL's parents will all someday go to DD. It's overwhelming. Even if she just inherited all my stuff, it would be more than she'd want or need. So we plan to sell most of the rest and put it away for college. My main collection and passion was Wedgwood Jasperware, but when I got to 50 pieces, I stopped. I figure that's a nice enough collection and I've no need to look further. (Although I could find the Waterford Wedgwood store in London blindfolded.

) As she gets older, if she doesn't care for it, I'm going to find out what relative DOES like it, and they will get it when I'm gone. I can't take it with me. I'd rather have memories than stuff.