After the discussion elsewhere in the forum about the 1 and 2, with vehicles seemingly dropping NIS and others appearing on route at odd times of the day, I thought I'd pay some attention. 

Noticed 22413 running light towards Saltburn. Hadn't seen it on the 1 or 2 previously, so tonight was the night I thought I'd pay the route some attention. 
The vehicle appears to have ran off Middlesborough locals, ran light to Saltburn and done one round trip on the 1.
Then ran light back to the depot.

Another, 36968 spent the day in Darlington and then ran light to Saltburn before doing 2 1/2 runs on the 2.