Can mobs in the Nether come to the overworld when there isn't an active portal in the Nether?

I recently built a portal room in my base. There was netherrack everywhere since I wanted it to feel like the player was really in the nether. I then lit up a portal but did not go in. However, after a few in-game days, zombie pigmen spawned inside the room. So is there now an active portal in the Nether after the spawning of the pigmen? I’m quite afraid since I’m building this on a Minecraft server with no rules(2b2t.org) and people can easily find my base and blow it up when they stumble upon an active nether portal since most new players usually do not destroy the portal in the nether. I walked a few thousand blocks after leaving the nether and as far as I know, there isn’t a nether portal at my base location. I don’t really know a lot about the game since I only purchased the game just a year ago and most of my knowledge comes from watching videos.