Sewage stink can not find source

[Fijifranny]

I do not know anymore.i bought a home.in spain.top floor(4rth floor)
I get a horrid stink in my apartment every time thebperson Below me does Laundry. Our washers are in the kitchen.
So i have taken the pipes apart and they were clean inside. The smell NEVER comes from the sink but always is wost inside the cabinet under the sink. The P trap is always full of water...so i can only think there is some leaky area after the p trap. I sprayed expanding foam everywhere..there was a hole in the wall where pipe goes into the wall..so i thought/ heard foam will make it airproof.
I just do not know what more to do. I can not check the roof vent cause ...well how?
I can ot trust any spanish plumbers..or any spanish anythin, cause service is so terrible here.
Plus they would NEVER be able to know where the leak is...cause it is not there all the time and once it comes,it stinks all over...so No Idea what to do???

 

[Aquarius]

Might be an open pipe somewhere, even in apartment below, smell will rise.

 

[Fijifranny]

Might be an open pipe somewhere, even in apartment below, smell will rise.

How can it get jnto the cabinet below my sink?

 

[Aquarius]

Because its a smell, which will be a gaseous product. I once attended a job, soil pipe was open to atmosphere in loft of a top floor flat, which you could smell it by the front door.