can you have a hidden on a battlefield you dont own

Yes — ownership doesn't matter, only control. Hiding places a card facedown "at a Battlefield you control" (Rule 422.1 / 811.1.b), and the battlefield's owner is irrelevant, so you can hide on your opponent's battlefield after conquering it.

The flip side: you can't hide anywhere you don't control — not your Base, not a battlefield you're merely contesting. And the hidden card is tied to control: if you lose that battlefield, all hidden cards there that don't share a controller with it get removed to their owner's trash (Rule 468.5.c), which is why you get a reaction window during combat but not after you lose your last unit outside of it.