What is the small nozzle type thing on the coolant overflow tank?

Please see the attached picture to see what I am saying.
So today when I checking under the hood and cleaning some leaves that were on the windshield, I knocked the cap of that nozzle off. I am unable to find it now. Should I be worried about it?

 

Thanks. Just joined the Prius family. The picture I posted is without the cap. It is a small black cap which goes into the area I marked with red. I got this reference on Amazon. The thing that has gone missing is the smaller black cap below the main cap.

 

Thanks. I will keep an eye on the cap to see what's going on. As an aside, is it OK to have the coolant level in the overflow tank, an inch below LOW level?

 

It just came off when I was cleaning the tank. It looked loose to me. Do I need to get a whole tank or just the small cap? I will try to see if anyone is selling on ebay. There is a similar nozzle on the power control unit coolant tank, and it doesn't have any cap at all. So I am thinking this should be OK. Still wanted to make sure as this is my first car. Thanks.

 

That's what it looks like when I looked at more pictures online. Will keep an eye on the tank to see any cooling is coming out or something.

 

Will try when I get back home and the engine has cooled down.

Just did a test with a wire and the only place the wire is going is the thread of the main cap. So I guess it's nothing major. Although, now I am curious as to the function of it. Anyways.

 

Thanks a lot Mendel. I plan on picking up coolant soon and monitor the levels to make sure there are no internal/ external leaks.