small vent

What about d). Perhaps someone temporarily announced routes for
unallocated space, setup a mail/spam server in that IP space, sent out
their mail, and stopped exporting the route.

Is there a place one can search to see a history of BGP announcements for
a given route?

That might show up as a flap, visible in the databases that keep and
publish histories of flap information. The IPMA reports at might help, specifically the
reports on Routing Instability and Routing Problems. There doesn't
seem to be an index into the data that would make a quick lookup based
on a specific prefix easy, though.

If I recall correctly, there was a raging debate a while back on the
topic of publishing route flap information because the Popular Press
was using such information to rate one provider over another, or to
assert that the whole net was about to drown in a sea of routing
updates. The many lawyers that subscribe to the nanog list asserted
that legal action against the publishers of flap information was a
virtual certainty, and then the thread ... just ... died.