Why are my crash logs disappearing?
System logs, for example, are compressed and then automatically deleted after a week. However, System Diagnostic Reports (whether it be a stall, spin, hang, crash, or panic report) shouldn't be automatically deleted. If you reset the NVRAM immediately following a kernel panic, however, the report is deleted (the NVRAM temporarily stores the report until reboot; if it's reset, the contents are erased). Short of forcibly removing the files (using Onyx, a similar app, or manually), this is the only situation I can come up with off hand as to why a report would vanish.
As a makeshift solution, however, you can save the kernel panic reports as a separate file.

From Console's menu bar, View &gt; Show Log List
Select the panic report (kernel_[longstring].panic) from the &quot;System Diagnostic Reports&quot; drop down
File &gt; Save a Copy As... &gt; save file to folder of your choice


UPDATE:
Onyx is the most likely culprit, however, the option for clearing logs/reports is located in multiple places:

Cleaning &gt; Logs &gt; System Diagnostic Reports
Automation &gt; Cleaning &gt; Logs, including Diagnostic Reports

Uncheck these options if they are checked.