I have two Chromium windows open, 0x31fd46b and 0x3072dc8 (this one):

jani@saegusa:~$ xwininfo -root -tree | grep Chromium-browser
0x31d2a67 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 321×216+275+321 +275+321
0x30fb2de ”Chromium”: (”chromium-browser” ”Chromium-browser”) 246×307+1232+305 +1232+305
0x3157fd9 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 246×307+36+124 +36+124
0x31fd391 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 1×1+-100+-100 +-100+-100
0x31b19f6 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 513×132+1011+38 +1011+38
0x30b5ffa ”Chromium”: (”chromium-browser” ”Chromium-browser”) 397×125+4+82 +4+82
0x3092643 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 321×216+791+829 +791+829
0x3030c84 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 306×230+587+441 +587+441
0x3030681 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 462×342+127+133 +127+133
0x3000027 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 200×200+0+0 +0+0
0x3000024 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 200×200+0+0 +0+0
0x3000001 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 10×10+10+10 +10+10
0x31fd46b ”Yleisurheilun Eliittikisat | Tv | Areena | yle.fi – Chromium”: (”chromium-browser” ”Chromium-browser”) 1014×438+2322+227 +2322+227
0x3072dc8 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 1659×50+94+83 +94+83
0x30000c3 ”Report a bug about “wmctrl” package in Ubuntu : Bugs : “wmctrl” package : Ubuntu – Chromium”: (”chromium-browser” ”Chromium-browser”) 1920×1056+0+0 +0+24

But wmctrl only knows about the one:
jani@saegusa:~$ wmctrl -l
0x01c00006 0 saegusa jani@saegusa: ~
0x02200003 0 N/A DNDCollectionWindow
0x02200004 0 N/A launcher
0x02200006 0 N/A panel
0x02200007 0 N/A Dash
0x02200008 0 N/A Hud
0x030000c3 0 saegusa Report a bug about “wmctrl” package in Ubuntu : Bugs : “wmctrl” package : Ubuntu – Chromium
0x03e00003 0 saegusa Rytmilaatikko
0x04200009 0 saegusa Kuvakaappaus
0x04600081 0 saegusa Synaptic-pakettienhallinta

This may be related to the fact that the other window has fallen out of view: it’s not visible on any workspace, but it’s still there as the audio’s still playing. It got out of sight due to another bug (probably in Compiz) which I have yet to report.