Infused a flat panel for the 1st part of my dash, and have got some small slit like marks in the resin.

Unfortunately they don't show up when I photograph them - very small little slits, 5mm long max, and about the width of a letter l.

I'm thinking I didn't shut off resin long enough before the vacuum perhaps ? and they are air bubbles ?

Any ideas ?

Definitely sounds like voids, but more likely caused by being a little on the lean side. Try clamping the vacuum line, then let the resin flow into the part for another 20-30 seconds. 

As Hanaldo said, it does sound like voids

Make sure you entire stack of materials has wetted through prior to clamping-off the resin feed, even if it means a little resin finding it's way into the catch pot.

I agree with the quotes below. I will add this to the mix. Always make sure your mold is warm to the touch before you infuse, not just room temperature. I place my molds in my over and get them warm before I infuse and it makes a big difference in reducing pinholes. I also heat my resin as well. This advice is from experience and not a try this and lets see what happens sort of thing.
Hope this helps you out.
Fred