According to an article in Ciel et Espace, there's a mystery in our galaxy.
Observing other galaxies, scientists have determined our own should know about three supernovae per century and yet, the last one was Kepler's in 1604. Another was Cassiopeia A in 1680, but it should have been visible in 1667 (making it even more atypical); (the 1987 one is not in our galaxy).
Both Kepler's and Cassiopeia A have released Titanium 44 which is considered a good indicator of past supernovae, but Integral, the Gamma ray satellite, can't spot any sufficient remnants of titanium 44 even in the plane of the galaxy (behind the dust thought to have hidden ancient observations of past supernovae).
Perhaps Kepler's and Cassiopeia are atypical and titanium 44 decay isn't a good supernova spotter. Then, theories must be revisited as to why so few supernovae are detected in our own galaxy.

Raoul