Fuel cap gasp

I've always been wondering why there's a "gasp" of air going back into the fuel tank when I remove the fuel cap. My daughter had a Vauxhall Astra once that used to do this and eventually, the tank imploded and I had to replace it.

This happens to both my F and the wife's F as well. I wouldn't mind but when I got my car it had no fuel in it, and it did it. Then I left it for a couple of days and it did it again. With no fuel and the engine had not been run.

Strange? What do you think?

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