Parked up today, key in ignition but not turned on, when a Disco parked behind me, driver got out locked his car via key fob and my tailgate window decided to fully open!!

Anyone else experience this?

No, but my first thought would be to see if my keyfob unlocks his car.
Did you try it?

I am sure that should read (but I'm a Manc)!!!!!

Not the same but kinda similar...
The bloke I bought mine off always locked it with the key so the alarm wasn't set. When I queried it he explained that there was a taxi driver next door and if ever he came home and used the taxi radio outside, it would set off the alarm on the Freebie!!
I wasn't sure that was the reason, but a few months down the line experienced exactly the same thing !
A taxi driver parked next to my freebie just as I was walking away from it after fetching some tools out, locking it as normal with the fob. Seconds later the alarm was blaring, so I asked the taxi driver if he had just been on his radio and he confirmed he had

Curiouser and curiouser.....

when taxi radios set the alarm off it is either a fault in the coax or the antenna has not been swr properly or the power is more than it should be , I used to have it but only of one taxi I stopped him one day and asked to see his radio set up and he had a coil in the coax because it was too long , I suggested that he take the coil out and spread the coax around the boot as it was nothing more than a inductor and his range or transmit and receive would be reduced , he did spread the coax out by the next time he came round this way , and knocked and thanked me for the info as now he was able to hear the base station a longer distance away and was able to get back to them. But as for the disco fob opening your rear window is possible as said you should have tried your remote fob on his.

Something similar happened to me when I first went to see the freelander. After coming back from the test drive and a final look around the alarm on the house next door went off And when the owner went to lock the car via the fob it would not lock neither fob would work. Then the alarm seized and the fobs then worked. Another time I called in to the local wicks store got out just as another guy was coming back to his car (a completely different vehicle to mine) and we both pressed our fobs and nothing would happen we both kept pressing the button and nothing. It was only when we noticed each other having the same problem we sussed it out.