2011 in question.
Does anyone know how the L322 detects a trailer is connected?

I had twin 7 pin sockets fitted to my vehicle.

I swapped the white 7 pin to 13 pin.

I also changed my trailer lights to LED type.

13 pin use pin 12 for detection, however mine is adapted to existing wiring so won't have that,

Usually 7 pin use resistance on brake light circuit. Trailer now has LEDs.

Can anyone confirm if L322 uses a switch in socket, or more likely just used resistance on braking (or other) circuit.

I have the two 7 pin sockets on mine (2012 VSE) and I use the black one for my bike rack.

If the car is switched on and the bike rack plug is inserted to the black socket, it pops up a message on the screen in the car asking if a trailer has just been attached.

Sorry - can't tell you which pin or how that is doing it. May be brake light resistance as my Thule rack seems to use standard Edison bulbs rather than LEDs. 2012 L322 TDV8 Vogue SE: Buckingham Blue
2005 Defender TD5 110 XS: Cairns Blue - Gone but not forgotten

I have single 7 pin. I don't get a message (at least I don't think I do!) when I plug it in but I do get the green flashing trailer symbol when indicating. Bish.

2005 110 TD5 CSW
2011 TDV8 Vogue SE (gone)

I will add a dropping resistor to the brake circuit and report back.

My car comes with a 13 pin connector and as my trailer is a 9 pin I have to use a pin converter and when I plug the trailer to the car I get the message on the screen

Just if this info is interesting for the purpose 2012 Range Rover 4.4 tdv8 Autobiography