Could this be considered and implied contract?

~I was arrested in 2016 and convicted of a felony drug possession crime. ~
I was performing really well at work and was selected to help lead the start up of a new department. I had worked for the company for 20 years and was in a supervisor role for about 14 of those years (in a leadership role for at total of 19 years). My work found out about my conviction this year and I was fired in May for non-disclosure. Our policy USED to state that we had to disclose all arrests, but it changed in the beginning of 2016 and clearly states that for my role, the only crimes/arrests/convictions that need to be disclosed are crimes of violence, theft, fraud, or breach of trust. So I didn't have to disclose it. When I was fired, I don't believe they knew the policy excluded my crime. They didn't even verify it when I told them it had changed.
~Being that I'm in an at-will state, they can fire me for whatever reason, but my question is this: If they set a trend for requiring "cause", are they creating an implied contract in that I can only be fired 'for cause'?
~In my 20 years at the company, 19 being in a leadership role, I have never even heard of someone being fired without cause. There has always been a clear policy violation that supported the action. I've been a part of many terminations and this was always the case. In fact, my manager, site director, and even HR commented during several employee discipline calibrations that we couldn't take the step of terminating because they were still within (usually barely within) our performance standards. In other words, there were many individuals throughout the years who were clearly a bad fit for the organization and toxic to our environment, but senior management and HR advised we couldn't terminate (even though we all wished we could) because there wasn't a supporting performance trend that was consistent within our policy to terminate.
~The entire supervisor staff had complained in several staff meetings that our policies were preventing us from being able to manage a positive work environment because we weren't able to terminate people who clearly didn't belong in the role.
~SO- if after 20 years of seeing that the organization will take action only for cause, why wouldn't I be able to expect the same in my situation? Would this fall into an implied contract? Why can't I reasonably expect them to show me a clear company policy that I violated to support being fired?