Render - a mysterious line but not a crack

We had the front of our terrace house re-rendered then painted last summer (18months). Very happy with it. However, a couple of months ago we noticed a thin hair line stretching from the edge of the upstairs window recess across to the edge of the render (and it seems to take a step up nearer the edge)

Had a good look out of the bedroom window and it isn't blown or even a crack as such - it seems to be just paint deep. It doesn't show in bright light - but is more obvious in dull conditions.

Is this the start of a crack, or is it just a join of sections of render or painting sections? A bit worrying and any light on this would be great - do we need to take action?

thanks a lot

Post a picture and we will give you some answers..

I would have done so but the line is too feint to show up on a photo. The description is very accurate though so any help welcome. Thanks

If it wont show on a photo I wouldn't worry about it till it does....