Here's a sample from my log:

Tue Mar 13 15:30:57 [initandlisten] connection accepted from 10.40.34.6:64663 #17859914
Tue Mar 13 15:30:58 [conn17859899] authenticate:

{ authenticate: 1.0, user: "user", nonce: "bdd36fd08133d104", key: "621c0399de14588aa23de270413fe2e4" }

Tue Mar 13 15:31:24 [conn17859899] warning: newExtent 4646 scanned
build index db_production.wi30495

{ _id: 1 }

Tue Mar 13 15:31:24 [conn17859899] build index done 0 records 0.405 secs
Tue Mar 13 15:32:37 [conn17859899] CMD: drop db_production.wi30495
Tue Mar 13 15:32:41 [conn17859899] end connection 10.40.34.6:64663

I logged in, created a collection with one record, dropped the collection, and then signed out.

NOTE that the connection identifiers don't match: 17859914 != 17859899

I have other examples of this same pattern in our log as well... I do not know however whether it has always been this way (for example maybe they matched at some point, but after a while it drifted out of whack).