Room 15903 (help !)

[whitbear8]

I'm stuck! When I get to 15903, I have one red key. When I open the secret door, I pick up a blue key and another red key, still leaving me with one red key. That's enough to get me through the first normal door, but not the second. Am I missing a red key somewhere else in the mansion? I've found them in 15702 and 16702.

I wouldn't normally ask but I've played section 4 around 20 times now and it's getting frustrating :-[