I have 80 detect as well and am able to reveal and get the notification that the active sigil is near by. Until I hear evidence that more than 80 offers an advantage, I'd rather put those skill points elsewhere.
To answer your original question Rico, yes, you must join VvV in order to participate in any part of VvV including stealing and returning the sigil.
If its quiet on the shard and only a few people are hitting the battles, you can manage without a single fight by stealthing about. When the shard is busier, some guilds are pretty good about laying fields and traps generously around the priests and altars making it really challenging to return a sigil or claim an altar.