Getting MAC Address from known network
Pending how long was the last connection, you could find the answer in your Console log.

I have it showing the information for last 7 days. Back to Dec 23. 2014

In Console type MAC in the search window.

Look for MacAuthEvent

In systemlog7.gz file.

EDIT:

If it is older than that:

Use Terminal and type mdfind systemlog

if it shows ....PreviousSystemLogs try that one. (highlight the find and use Reveal in Finder)

If it does not show it in Terminal then look for it in the hidden folder /private/var/db under .cpgz archive that you have to unpack.