BobVila.com

You might want to have a home inspector go over the house. Both in the current stage and several steps as it is completed.

You will want to get one that does new home inpsection, not just old homes. They are reqly much different inspections.

As to your question.

First (and hopefully) the stain might be from much earlier in the construction.

But vinyl siding is NOT WATERPROOF. There should be a weather barrier on the house. That can be either house wrap or felt paper (tar paper). Along with proper flashing of the doors and windows.

All of that should be done before insulation and DW.

While the plastic might keep the water off the drywall. It does not keep it out of the insulation, studs, sheathing and bottom plates.

And infact the plastic helps keep the wall from drying out. Thus you can have mold and rot in the wall and not know about it until VERY SERIOUS DAMAGE IS DONE.