anyone know where the custom of putting a cross on a grave came from?

Answer 1

Early Christians used them in the Christian catacombs in Rome in the second and third centuries.

Here is a nice article on the Archeology of the Cross and Crucifix: http://www.newadvent.org/cathen/04517a.htm

With love in Christ.

Answer 2

It comes from Mithras. Mithra was regarded as created by, yet co-equal with, the Supreme Deity. Mithraists were Trinitarian, kept Sunday as their day of worship, and their chief festivals were what we know of as Christmas and Easter. Long before the advent of Jesus, Mithra was said to have been born of a virgin mother, in a cave, at the time of Christmas, and died on a cross at Easter. Baptism was practised, and the sign of the cross was made on the foreheads of all newly-baptised converts. Mithra was considered to be the saviour of the world, conferring on his followers an eternal life in Heaven, and, similar to the story of Jesus, he died to save all others, provided that they were his followers.

Answer 3

Very interesting.
I am coming back to this question to see what the answer is or if anyone has a link.

Answer 4

It was used to tell early grave robbers where to dig.