The applicable Technical Directive only came into force this race. It is possible Renault could have heard the Haas team was considering breaching the regulation somehow, but from what I gather, they only managed to get the incriminating evidence on the starting grid, hence why it took until the post-race protest window for the matter to be investigated (you can't really do a technical inspection of a car floor that's on the track and running at race speeds).
While it might have been wise for all the floors to be proactively checked on the Thursday, they weren't. Perhaps there was disbelief that Haas would actually go through with their proposal to deliberately not have the correct floor on the cars (wouldn't be the first time the FIA has failed to take someone seriously when it should have done).