Topping up coolant last evening I happened to run my finger down the neck of the coolant expansion tank. Was pretty surprised to feel a distinct indentation in the surface right where the cap's o-ring sits.

Comparing with my Polo there's a just-detectable one on that too, but the Fabia's one feels big enough to possibly affect the o-ring's ability to seal as intended. 

 

Don't try this unless the engine is cold, but have a go yourself and see if yours has a similar 'print' of where the o-ring's been sitting for the last 13-20 years.

 

Little vid showing the highly technical experiment in action:

 

 

 

If there is no need to make frequent top up and the pressure builds up i think should be ok

How to know what level the pressure is at though?

If it were not a good seal then you would see coolant loss for sure.

Fair point. 

I wasn't particularly thinking that it was definitely leaking, just a bit surprised to find it, and was hoping others might explore for the same thing on their - perhaps older - Fabias (or any other VAG cars with the same bottle) to see if this is a common thing. At some point it's going to fail to seal fully, if the dent keeps getting deeper.

Just checked, have it too