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In the episode Sock Opera toward the end, Grunkle Stan starts to record Bill Dipper and Mabel fighting. However, then Bill leaves Dipper's body and make a brief millisecond appearence all in gray, before Dipper's ghost can reenter his body. Well, we don't know when Stan left the building and/or if Stan saw the Bill Cipher part.
It's likely that they stayed in their seats alongside with the others (Soos, Wendy, etc). So, couldn't Stan of seen Bill?