Thanks to all the tickets provided by Leonardo, AzuiSleet and I have managed to discover what most of them pertain to.
I have updated the OP with version 0.5.1 that should handle most of these correctly now, there may still be some errors from clients sending purposefully invalid tickets to try and exploit a SRCDS crash bug that was patched a long time ago.
SteamTools will still log it's ticket handling extensively, except all non-fatal messages will be in the normal SM log instead of the error log.
Here are the 4 ticket messages you may see:
Client connecting from <IP Address> isn't using Steam.
Client connecting from <IP Address> (<Steam ID>) is in offline mode.
Client connecting from <IP Address> (<Steam ID>) was denied by Steam for an unknown reason. (Maybe an expired or stolen ticket?).
Client connecting from <IP Address> (<Steam ID>) is in offline mode but their ticket hasn't expired yet.
The former 3 are all denials from Steam, the 4th is the same as the 2nd except that Steam said it was OK for them to play.