The 100% in
100% polyester and 100% silk suits refers to the visible part of the suit.
Underneath that is an entire world of stuffing, reinforcement and other junk. The easiest place to see some of these is under the lapel; under the felt is something stiff, and it's not silk/polyester.
These hidden parts are made from whatever material the factory can buy at the cheapest price. It usually differs even from one identical suit to the next; depending on what came out of the sack next.
Shaatnez checkers will tell you that it's not at all unusual to find both wool and linen in these pieces, maybe even material made from both.
There's also the issue of the thread used for sewing on the buttons. Linen thread is still one of the strongest threads available.
BTW: Have you ever seen a zipper of 100% polyester or 100% silk?
To answer your question: There is only one Safek:
Do the non-silk/polyester sections contain wool and linen?
Disclosure: My father was a Shaatnez checker for over 30 years.