where is it exactly? if it's on your own property you should be fine, but if it's in a park or public trails you will probably be noticed. i've done circles inside and outside and it's better to stick close to home. [a big back yard is best] if you wish to use it as your sacred space you can, just don't leave your stuff their. even covens who preform outside don't typically leave tools outside. a rock circle or natural altar [stump, big rock] fine but not candles, BoS, athame and so on.
you could try casting a protection spell on the area, moving some brush onto the path to the space, or maybe set up some little things to scare people off [fake cobwebs for example] but if you're doing things off property people will notice eventually. i have cast circles off my property twice and both times it didn't end well. the first time my coven decided to hold a Beltane circle in a wooded area in the park and a family who lived adjacent to the park threatened to call the cops on us. the second time was in a field behind my apartment and a group of teens walked through the circle and tried messing with our altar. trust me, while outside is good, make sure it's your own property.