Earlier movie in series?

When Aline MacMahon's nurse and Guy Kibbee's detective first confront each other there's a lot of wisecracking repartee that seems to show they've met on a previous case. The review by Ron Oliver says "The MacMahon & Kibbee characters reappear in THE PATIENT IN ROOM 18 (1938) and MYSTERY HOUSE (1938), although portrayed by other performers."

But did they appear in another film, possibly now lost, made before While the Patient Slept? A search under Mignon G Eberhart (the author of the novels in which the characters appear) doesn't turn up anything useful. But I can't see why Warners would leave in the dialogue about their previous cases unless they had filmed one. Well, now I think about it one possibility does come to mind: Warner Bros. might have had their writers dash off 2 or three scripts at once, with the idea of making a series pair (like Hildegarde Withers and Inspector Piper over at RKO), and then deciding to film only one of the scripts, choosing the one they thought would make the most attractive movie rather than the first in chronological order.

But if anyone knows the answer to this not especially burning question, I would be glad to read it.