In the old days, your company could not easily access the records of a non-affiliated carrier...they would not have any electronic connectivity. It's possible that a chief pilot-to-chief pilot phone call would have resulted in shared info...but they would have had to call a lot of airlines unless they already knew which one you used!

Today we have CASS...everybody has a access to a common system. Every CASS lookup generates an audit trail...I don't know for certain, but strongly suspect that any CASS carrier can quickly pull up that audit trail for it's own pilots. This would not prove you flew, but rather that a gate agent CASS-verified you...this would still be a large smoking gun.

Basically unless your company is suspicious of you, I doubt they would have any reason to look at your CASS records. I suppose it's possible that some really obnoxious companies routinely research the JS/nonrev histories of pilots who call in sick, but I have never heard of that.

If you are in some sort of conflict with your company, better assume they can obtain any info they want and play it safe.