No corrosion was found during last year's annual, this time the grinding took off a chunk
Corrosion will exist, and indeed worsen faster, under unbroken paint. When the paint is scraped, a chunk of it flakes off, and corrosion is visible. That corrosion has been growing for years, but "suddenly" becomes apparent. It's not new corrosion, it's just newly noticed.
In this photo (PA-28-180 spar), there is exfoliation corrosion on the top edge of the spar, which is blatantly obvious. To the left of the bulkhead fitting, corrosion is obvious. But, the bubbled paint all the way around the fitting is also corrosion, just the paint hasn't flaked off yet. This spar was scrap when I inspected it, and being a 1967 Arrow, with other rust, and lots of other corrosion, it was not worth any attempt to repair. Had they attempted to repair the corrosion in this photo, taking the wing apart would have revealed lots of other corrosion. The plane was scrapped.
PA-28-180 wing spar corrosion