I have investigated and timeout event not emits, in this case:

for poolSize = 5, socketTimeoutMS=5000
1. connect to mongo and do many requests
2. mongo client open 5 sockets and nice work with them
3. block network to mongodb server
4. all sockets timed out not at the same time
5. In my case, 4 sockets timed out, event not emit because we have socketCount() > 0
6. after 1-2 seconds 5th socket timed out too, but socketCount() > 0 and timeout not emits, because we have 4 connecting sockets