Officers want to trace the driver o fa car who was involved in a confrontation with another motorist on Tuesday at around 12.30 pm.
It happened on the B7076 APR road at the junction with the A701 road. A 68-year-old man from Lockerbie was sat in his blue Audi A4 estate, after breaking down at the side of the road.
A car, described as a burgundy Vauxhall, drove alongside, stopped, and the male driver took a photo of the Audi.
The man then appeared to mouth something before driving off.
However, a short time later the same car and driver returned, and at the same time a window was smashed in the Audi as the burgundy car drove passed, then sped off.
Constable Stephen Glendinning at Lockerbie yesterday said: “This does appear to be a bizarre incident and we are at a loss to understand what the motive was for this behaviour.
“The driver of the burgundy car is described as being in his late 30s/early 40s, medium build, had a full black beard, possible black hair and was wearing a black coloured hat.”
He appealed for anyone with information to call Lockerbie Police on 101.