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The Simlah Chadasha (Siman 18 Seif 16) writes that if a knife with a Chazaka was held unused for three days - and one then slaughtered with it (without checking if it is smooth) and then lost it, that knife no longer retains it's Chazaka, as we are afraid that it may have gotten rusty, and the meat is forbidden.
Does this still apply to modern-day "Stainless Steel" knives?
Yes, this would still apply to modern day knives. The Mateh Asher, Number 46, Brings the reasoning for this halacha from the Tevuat Shor. The Tevuat Shor is by the author of the Simla Chadasha, where he brings the reasons for his rulings.
There he explains that when Shabbat preceded a two day Yom Tov, Halachically, one may sharpen and check his knife on Erev Shabbat and rely on that checking to slaughter with the knife on the second day of Chag, 3 days later. We therefore know that a knife retains its assumption of sharpness for 3 days. However, we have no Halachic proof that it retains its sharpness more than 3 days. Therefore, after 3 days it looses its Chazakah.
If so, there is no difference between ancient and modern knives.
