Why not entering is the whole instruction
A conscientious cleaner will want to check. That instinct is understandable and it is precisely the wrong one, because the two possible situations require opposite responses and there is no safe way to tell them apart from the doorway. If the building was left open by the last person out, nothing is lost by waiting fifteen minutes. If it was not, entering is the worst available decision.
The same applies to closing the door. Securing a building that somebody is still inside is not helpful, and moving anything at the entry point may disturb something that later matters. The instruction is to leave the situation exactly as found, move to a safe position, and make the calls. Everything else follows from a decision made by somebody with more information. Leaving everything as found costs nothing and preserves every option.
