mongocryptd launched with the only argument of --idleShutdownTimeoutSecs=60 creates a socket file in /tmp and a pid file in the current directory:

2020-01-30T18:21:16.802-0500 I  CONTROL  [initandlisten] Using lock file: /home/w/apps/ruby-driver/mongocryptd.pid
2020-01-30T18:21:16.818-0500 I  NETWORK  [initandlisten] Listening on /tmp/mongocryptd.sock

Since both files are associated with the same process, they should be placed in the same principal location (either global to the system or local to the application launching mongocryptd/current directory).

As I understand it, mongocryptd is intended to be system-global/shared; if this is correct, I expect the pid file to be placed in a system-global/shared location as well (e.g. /var/run on systems obeying FHS, see http://www.pathname.com/fhs/2.2/fhs-5.13.html).

Tested on 4.2 server built from source recently.