While PCs are often meticulous in detecting for magical powers or amazing abilities in whatever gear or gadgets they find, sometimes you can get away with slipping something in unnoticed. If you make it something otherwise forgettable, it helps.
Imagine a magic potion flask. If the PCs detect magic, of course it gives off an aura of magical power, but then they expect that from the potion inside anyway. Nobody suspects the flask itself.
By the way, what do PCs do with the flasks when they drink a magical potion? Has anyone ever asked this question before? The players usually just cross/erase used potions off their equipment lists. Where do the flasks go? Are all dungeons littered with the empty potion flasks of previous adventuring groups, like discarded cans and bottles by the side of the road?
Anyway, make a point of having the PCs find all their empties when they make camp and take stock of their gear. What are they going to do with these empty flasks? If they're like most adventurers, they might try to sell them, or perhaps they'll fill them with holy water or something. Here's where you have the magic of the flask step in. Maybe it always returns to their pack, no matter how they try to get rid of it. Maybe it converts whatever it contains into the same potion that was originally found in it (once per week, say, so it's not overpowered). Maybe it does something else completely, like attract monsters, or prevent any fires from being lit within 30 feet.
Who would suspect a flask?