I ran this test on MongoDB 3.4.15 with the following config string:
file_manager=(close_handle_minimum=1,close_idle_time=5,close_scan_interval=10)

Test:

1. I inserted a document each in two collections, hence creating these collections in the process
2. I dropped these two collections

Behaviour that needs investigation:
Even though the sweep server is configured to sweep every 10 secs and close handles if there are any (close_handle_minimum=1), after several minutes the dhandle count has not reduced to what was before creating the collections.

In the t2 data above:

• 2 creates at A, 2 drops at B
• Several minutes after the drops, after several data handle sweep attempts, active handles has NOT gone down.