Reimu looked at the factory again, still unsure what she was looking for, exactly.  But, this was the only lead she had, so she didn't have much of a choice.  Still, not much point in finding a hidden entrance.  Might as well just charge through and hope she could find something useful.  "Charge through" being figurative, of course; she tried to be as stealthy as possible while entering the back door, not wanting to get into a fight she couldn't handle.