Listing network hardware - why does Ubuntu sometimes hide eth0 until it is connected?

On my laptop, I have Ubuntu 14.04. This laptop has both an RJ45 connector for wired Ethernet, and wireless/WiFi. I have noticed, however, that on this laptop, right after reboot, I have only this:

$ ifconfig
lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:608 errors:0 dropped:0 overruns:0 frame:0
          TX packets:608 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:76086 (76.0 KB)  TX bytes:76086 (76.0 KB)

wlan0     Link encap:Ethernet  HWaddr XX:71:XX:XX:b1:XX  
          inet addr:YY.0.XX.XX  Bcast:YY.0.XX.XX  Mask:255.255.0.0
          inet6 addr: fe80::XXXX:XXXX:fe3f:b1db/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:2589 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2044 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:1458453 (1.4 MB)  TX bytes:292837 (292.8 KB)

In fact, if I turn off wireless / uncheck "Enable Wi-Fi" from the Network Manager applet, I have only this:

$ ifconfig
lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:1794 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1794 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:178068 (178.0 KB)  TX bytes:178068 (178.0 KB)

As you can see from the above ifconfig dumps, eth0 corresponding to the wired ethernet network is not listed at all! And in fact, it won't appear, until I connect the laptop with a router/switch, and the laptop gets an IP address!? In fact, while there is nothing connected to the RJ45 connector, the laptop reports just this via lshw:

$ sudo lshw -c network
  *-network               
       description: Wireless interface
       product: AR9485 Wireless Network Adapter
       vendor: Qualcomm Atheros
       physical id: 0
       bus info: pci@0000:01:00.0
       logical name: wlan0
       version: 01
       serial: XX:71:XX:XX:b1:XX
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress bus_master cap_list rom ethernet physical wireless
       configuration: broadcast=yes driver=ath9k driverversion=4.2.0-42-generic firmware=N/A ip=YY.0.XX.XX latency=0 link=yes multicast=yes wireless=IEEE 802.11bgn
       resources: irq:16 memory:f7c00000-f7c7ffff memory:f7c80000-f7c8ffff

Notice, the wired/eth0 adapter is not listed anywhere ?!

Otherwise, when I've connected the laptop's RJ45 connector with a network cable to a router or Internet cable, I get an ifconfig entry like:

eth0      Link encap:Ethernet  HWaddr XX:XX:2b:XX:XX:XX
          inet6 addr: fe80::YYYY:YYYY:fe0a:YYYY/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:6 errors:0 dropped:0 overruns:0 frame:0
          TX packets:61 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:1980 (1.9 KB)  TX bytes:12108 (12.1 KB)

In fact, in the above snippet, the eth0 didn't even get an IP4 address assigned; so apparently all it matters is that it is connected somehow to an Ethernet network.

Note, on this laptop, /etc/network/interfaces was containing just:

# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

... however, I added auto eth0 as a last line, saved and rebooted - and there was no change in behavior whatsoever (i.e. eth0 was still hidden, until connected).

Now, on the other hand, I manage a desktop machine as an Ubuntu server (also 14.04), and on it I've always seen eth0 show up in ifconfig - even if I have pulled the network cable from the desktop's RJ45 connector?!

So my questions are:

• Why does Ubuntu sometimes "hide" eth0 (or other network interfaces) until they are connected - and why is it not consistent (i.e. on desktop it is always shown, on laptop it is not shown until connected)?
• How can I list all network hardware, on an Ubuntu that "hides" network interfaces like this - as in that case, as shown above, not even sudo lshw works?