Zombies found on minecraft server with mobs disabled

I'm pretty new to Minecraft having only just bought the game about a week ago, so apologies if I'm missing something obvious here (I've attempted to search for the answer, but didn't find any relevant results)

I've got a private minecraft server on my home network in survival mode with spawn-monsters disabled and normal difficulty, I've not seen any hostiles, or at least not until just now. I've just found a location in the world, deep underground, where I've found what was a fully enclosed pitch black room that was full of zombies. I got overwhelmed by them within seconds, but I did briefly notice a number of storage chests and other items within the room.

If it is relevant, I found the room by accident when I saw my first moss cobblestone. Not knowing exactly what the block was, I mined them thinking I'd found another resource, in doing so, this room was revealed.

I'm the only person that has played on the server (it's behind a firewall, and I stop and start the server on demand).

Is there any explanation for this? I didn't expect to see any hostiles at all with spawn-monsters option disabled. I'm going to head back as soon as I get chance, but was wondering what it might be?

You found a Dungeon. These contain Monster Spawners.

The spawn-monsters multiplayer option in the server's config file only controls natural, random monster spawns in dark areas. Unless the server difficulty is also set to Peaceful (or 0), Monster Spawners will still be active if you go near them.

What you've found is a Dungeon. Specifically, a Zombie dungeon. It contains a Monster Spawner which is able to spawn monsters even if spawn-monsters is disabled. The only way to stop a spawner from spawning monsters, is to fully light up the area, or set the server to peaceful.