So, I was farming ATK and DEF EVs (Route 31) on my swinub and i didn't noticed when my item disappeared. I checked my backpack and I don't have it.
I was killing Sentret, Nuzleaf, Shiftry, Sandshrew and sandslash , all of them had 4-6lvl and any of them can learn covet or thief on that low lvl.
I wasn't trading or anything, so i don't have any other idea what happened