Ben enters the restroom, checks for feet under the stalls, then heads back out. As he's gone, a pair of feet appears in a stall and Greg comes out of his hide.
Questions to this:
Is "checks for feet under the stalls" okay? Or should it be "stall doors"?
Is "as he's gone" grammatically correct? Or should it be "as he's left"?
A "stall" is the booth itself, not the door to the stall. So I'd suggest it should be "stall doors".
I prefer "once he's gone" (which would suggest Ben's left the restroom and is moving down the hall (or wherever)) or "as he leaves" (which suggests the feet appear just as Ben is passing back into the hall (or wherever) from the restroom). But also, feet wouldn't appear in a stall, unless they're some sort of magic item...I think that needs a re-word - maybe "a pair of feet become visible under the stall door" - I think it depends from what angle you want the reader to visualise what's going on. Also, I think "a pair of feet appear" sounds better than "a pair of feet appears" - that doesn't appear to be correct to me.