Heap profiler shows memory usage increasing over time on the secondary. The profile is characteristic of a buffer that grows by doubling each time.
Reproduced by a high rate of collection creates and drops using mapReduce directed to the primary:
In this repro the mapReduce commands are done on the primary, but the memory increase only seems to happen on the secondary.
I don't know whether it is specific to collections created and dropped by mapReduce, and I don't know if it's a 4.2 regression (but I note there appear to have been some considerable code changes in this area).