Sounds like the typical sealer primer used as a final coat before paint. We typically see this applied to only exterior facing surfaces. So, say your hood was red-oxide primer on top of bare metal, top and bottom. The gray sealer would be applied to the top and then exterior color applied to both sides.
Some assembly plant installed parts after the gray primer, so we sometimes see gray on the rear quarter panels when the end caps are removed. Or a line of gray behind the edge on the tail-light panel where the rear valence attaches.
It is also typical to see the pot metal parts primed only with this same gray primer.