Help please

Went to MIJ performance exhaust yesterday and had a mid sport exhaust fitted which i am very pleased with..but..after getting home my neighbour popped round to tell my hazzards were flashing..so i pressed the remote and checked all the doors and boot were locked then went inside..
About 11pm just as we were going to bed my alarm went off so i hit the remote and it stopped....2am this morning it went off again..not happy....so this time i left it unlocked as its on my drive..

after looking in the boot this morning i have found that the panel that covers the batterey has what looks like a brown earth wire attached to it which has been broken...now i know when they were doing some welding/fitting the new exhaust the man told me they dont disconet the battery they use some red thingy on the leads which stops my electrics getting fried and also saves your cars settings.....but it looks like they snapped the wire they took the cover off in the boot for access to the battery

I have reconnected the wire and it has not gone off once today at work...so im hoping it wont go off at 2am again...
But has anyone any idea what this brown lead is........??????