In the past month alone, shadowy portraits have been found hidden in longstanding masterpieces by Titian and Picasso. What can they and other such discoveries tell us?

Something's stirring. Every few weeks, it seems, brings news of a sensational discovery in the world of art – of paintings hidden under other paintings and vanished visages twitching beneath the varnish of masterpieces whose every square millimetre we thought we knew. This past month alone has brought to light the detection of mysterious figures trapped beneath the surface of works by Titian and Picasso. But what are we to make of this slowly swelling collection of secret stares – these absent presences that simultaneously delight and disturb?

In early February, it was revealed that researchers at the Andreas Pittas Art Characterization Laboratories at the Cyprus Institute, using advanced imaging and a new multi-modal scanner combining different techniques, had proved the existence of an upside-down portrait of a mustachioed man holding a quill beneath the Italian Renaissance master Titian's painting Ecce Homo, 1570-75. On its surface, Titian's canvas portrays a bedraggled Jesus, hands bound by ropes, standing shoulder to shoulder with a sumptuously dressed Pontius Pilate, the Roman governor who will sentence him to death. What is this strange, erased, anachronistic scribe doing here and what is he trying to tell us?

The presence of the hidden portrait, who peers imperceptibly through the craquelure – those alluring cracks in old master paintings – was first described by the art historian Paul Joannides and its significance to the surface narrative is more than incidental. While the identity of the topsy-turvy figure has yet to be determined, it is clear he helped shape the wrenching composition under which he has been buried for the past 450 years. The analysis of the materiality of the painting's layers in Cyprus has shown that the contours of the hidden figure's face dictated the curve of ropes binding Jesus's hands – establishing notes of harmony between the successive and seemingly contrary compositions.

That sense of quiet collaboration between layers of paint – between what is there and what used to be there – is more striking still in the hidden countenance of a woman found by conservators at The Courtauld Institute of Art beneath a painting from Pablo Picasso's Blue Period – a portrait of the artist's friend and sculptor Mateu Fernández de Soto. Also discovered with the use of infrared imaging technology, the portrait of the as-yet unidentified woman is rendered in an earlier, more impressionistic style, and appears, when brought to the surface, to be whispering into de Soto's ear, as if the past and present had merged into a single suspended moment.

In most instances, these buried portraits are merely the ghosts of rejected compositions that we were never intended to see – and could not have, were it not for the aid of advanced imaging tools that allow experts safely to peer beneath the paint without harming a work's surface. X-rays uncover hidden sketches, while infrared reflectography is capable of exposing subtle details masked by old varnish – details which, once glimpsed, are impossible to unknow. Once revealed, these portraits demand to be reckoned with. What follows is a short survey of some of........

© BBC