Today's surplus-of-old-mysteries thought: it's noticeable how many of these:
a) would be solved trivially with tools of the modern era (phone logs, CCTV, DNA); but also
b) *are* solved by investigative methods that just seem ludicrously implausible now (eg routinely recording serials of banknotes)
Makes you wonder what the verdict in 2070 on contemporary mystery novels will be. "Strange to think of an era before we could check car route-finding logs"