Legal Question in Employment Law in Washington

Re: I was fired from a job that lasted less than 90 days.

First, your legal question; I'm not aware of any law requiring the employer to explain why it has rescinded a job offer. Sometimes, it would be a good idea, particularly if the employee might infer an illegal reason.

Your other questions do not have to do with the law, but I'm a good one for spouting off with opinions, so I'll answer anyway. Yes, it is quite possible that the new employer found out about the short term job from which you were fired. Personally, I would recommend including it on the resume, though others may disagree. If the prospective employer learns of it by other means, you are going to look much worse than if you disclose it. Also, if the prospective employer asks you to complete an application that asks you to list previous positions, you are misrepresenting yourself if you don't include it. But, if you do include it on the application, but it's not on your resume, that's not going to be helpful to you either. No employer is going to appreciate feeling misled, so, as a former employer myself, I feel your chances are better if you disclose than if they find out on their own.

Good luck.