Unknown disconnected tube in the engine found

Hey guys. I have a seat ibiza 2001, and when I was checking the oil level, I found this tube that was disconnected from the side of the engine. No idea what it is or what its use is, i dont have the owner's manual and cannot really go at any mechanics at the moment. Was wondering if anyone can help me figure out what it is. Here's a pic below. The blue spot would be where it connected I assume as it had a small empty hole on that space.