I have a 2011 Nissan March, currently experiencing an issue which cannot seem to be resolved.

A few months ago I noticed that when the gearshift is set on drive (D) or reverse (R), the gear sign on the dashboard starts flickering / flashing. At times it does this at random but other times it seems like it is triggered by either the blinkers or the break paddle. 

I had my car checked by 2 different mechanics but the computer doesn’t seem to be registering any episodes / faults. A few weeks ago I also noticed that the engine warning light was on - had this checked aswell but nothing came up on the computer. 
 

Has anyone experienced something similar?

That definitely sounds like a poor earth somewhere in the engine bay area.....check the earth lead from the battery and any that may be attached to the engine and body

Also on the offchance, it may be something simple like a bulb.....check the rear indicator and tail lamp bulbs are the correct type as sometimes if incorrect they can backfeed current through the electrical system and create faults like you describe