EPROM memory - the theoretical possibility of recovering data from memory after erasing the exposure with ultraviolet light?
Dr. Sergei Skorobogatov from the Computer Science department of the University of Cambridge has written a rather sufficient article on this matter. In his research, he writes:


  Even after an erase operation, the transistor does not return fully to its initial state, thereby allowing the attacker to distinguish between previously programmed and not programmed transistors, and thus restore information from erased memory.


in his abstract.

You may further investigate why in this research article as he provides physical depth of an EPROM module along with his methods of experimentations.