This is on a debian testing system.
xfce4-power-manager is configured to always show icon in systray.
The battery is unplugged.
before suspend to ram, everything is OK. Here are the operations that make it behave strangely :
1) suspend to ram
2) resume => the icon is not shown anymore.
3) xfce4-power-manager-settings => the setting is still to "always show icon"
4) set it to "when battery is present" then again to "always show icon" => the icon is back in systray
5) move mouse pointer over the icon => the icon disappear again !