While debugging a cluster with degraded performance I noticed very high lock usage and disk utilisation. Only when I looked at db.currentOp(true) to see ALL queries running (including background threads and such) I found the TTL query.

It would be nice if this query would be revealed at db.currentOp() (without the flag) and more importantly, the deleted documents will be revealed in serverStatus and therefore mongostat.

I understand this might not be a bug and it works as designed, and serverStatus contains the number of deleted documents through TTL under metrics.ttl.deleted but it's still confusing.