its possible to undermap, but more commonly people just follow behind you into your base and you just don’t see them, either b/c you’re not paying attention or they don’t render in due to client/sync issues. The intruder then finds a place to hide out of sight and when the time is right start de spawning everything they can access. This is an insidious but valid tactic that my tribe has done more than a few times to bring enemies down without them even knowing we were there.
For example: one time we snuck into a base following behind a very tired enemy player just finishing up a brimstone run. We log off in a hidden spot and wait a few hours. Then just before raid window we log back on, start pop corning stuff and notice an arch priest was recently left in a t3 alter bubble with enough zeal to pop a god coin. We crafted a god coin, jumped out and then godded the very same base.
Another time we snuck into another base and slot capped our inventories with everything we could grab and then crafted midnight grove potions to teleport out. The enemies never figured out how we pulled that one off but accused us of all sorts of sorcery.
TL;DR PVP in Conan can be very savage and not all enemies raiding your base announce themselves.