There is a very small rise on the standard (full or compact) USP barrels that you might mistake for a place to hold the o-ring?
The o-ring notch is definately a relatively deep groove and hard to miss. You would know it if you have seen the tactical/expert barrel without the o ring.If your compact barrel has an actual groove, then it is either aftermarket or altered. They are not factory produced to my knowledge.
If it is grooved, you might want to consider purchasing some spare o-rings to ensure the lip of the groove is not damaged by the slide as the barrel is relased from lockup. A "just in case" thing... it might tighten up your groups a little.
