If an employee was asked to resign for suspicious activity or was fired for stealing could we call their loans even if they are NOT past due?

Do you have a demand feature, or some addendum for employee loans that says the loans are due, at the bank's option, upon termination (voluntary or involuntary) of employment? If not, then IMO no if they are not in violation of the loan agreement.

Even if you have a demand feature or the addendum, under the circumstances you describe I would consult legal counsel before making any demand.

After looking at the agreement I do not see any language to that effect. It does say that "I will be in default if:
(7)I do or fail to do something which causes you to believe that you will have difficulty collecting the amount I owe you." Then the remedies says I may demand immediate payment... That may be streching it though.

Based on the activity you dismissed this employee for, this may not be stretching it. Again, contact your counsel for advice in this situation and be sure to point out that section of the loan agreement.