Are “frequent and regular” inspections of your construction sites made? If not, OSHA could write a citation under 1926.20(b)(1) which says that it is the responsibility of every employer to do so. What proves that an inspection was done? OSHA doesn’t require written documentation but it has this mentality that unless there is written proof, one wasn’t.
Every contractor should be aware of OHSA’s “Multi-Employer Worksite Policy” which directs enforcement officers to find a “controlling” employer as well as the “exposing” employer. If you receive a citation as that controlling contractor one way to make an argument that you were not aware of the hazard is to pull out the written audit reports that prove you do frequent and regular inspections and if such a hazard was not noted on the report you might have a chance to get out of the citation.