Hi Chris, these markings remind of a stone I found at the Conjure cairn, have a look there to see if that is helpful. As for me I drive past Daviot thru the week so I'll go and have a look for myself.
I looked at your photos and have seen similar marks myself... think it was at Tyrebagger. I'll have to check my photos.
At Daviot if you go in through the gate, walk straight ahead until roughly level with the centre of the circle then turn left. This stone is about halfway to the fence, forming a little hump and quite visible. Probably nothing but I'd like to get a second opinion!