Should I be suspicious of meat/seafood on sale?
In the case of quickly perishable items like meat, when it's not an advertised sale, shops will sharply discount prices if they anticipate that they will be otherwise left with product beyond the sell-by date. So the answer to your question is both yes and no.

There is nothing wrong with buying something just before the sell-by date, but you do need to use or freeze it sooner rather than later. For everything but seafood, I jump all over those sales. I can barely tell the difference between super-fresh and right-at-the-sell-by-date for most items. Seafood is an exception for me, but that is just me. I will pay extra to get super-fresh seafood, even though I'll buy milk and meat right at the sell-by date if it saves me a few pennies.

So in this case, like many others, the key to making an informed choice lies in reading the label. No reputable grocery will alter the sell-by date. At least in the US, getting caught doing that will close a shop down and land people in jail. So read the label (or the tag on the meat-case window), and be informed about what you are buying.