I can see a small thing which looks like copper in the middle of the cap, but im not sure of any seal, i cant see anything, but would i see it?

And coolant could leak out even if i had the cap screwed on tight?

I havnt noticed any coolant leaking from the expansion tank, and ive never seen any fluid on the floor after parking up, unless it only comes out when i am driving which i wouldnt see.