Mine has a plug in it. I've always wondered what that hole is for.
Thanks for verifying dmagerl! Any issues with outflow pressure and what size tank are you using it on?
I'm on the edge of purchasing a new xp3 but worried that the flow might be the same as the current one I have. I can't compare this filter because I never had one before but if people are using it on 75g + I'm sure there should be plenty of flow for a 40g. The spraybar is useless, and even though I put it about 4" below the water surface the flow barely trickles the top.