In reply to Porky:
It looks like incised lettering in a roman font, certainly nothing like the flowing symbols and patterns of Pictish carvings. A bit more contemporary in age I imagine.
Pity you didn't get a pic with a more oblique light source, it would have picked out the details/shape far better.
If you can remember the exact location, it might be worth checking out the fantastic canmore.org.uk searchable map. Plenty of sites listed on Pabaigh.