Any divorce can be messy, but some are especially messy if one or both parties are trying to hide things from the other. That makes the process fraught with confusion, betrayal and mistrust.
Hiding Assets Is Fraud
It isn't uncommon for the spouses in a divorce to try hiding assets from each other (and from the other party's divorce attorney). This is a type of fraud, so it is often investigated by your divorce lawyer the same way fraud is.
As an analogy that your divorce lawyer might use is this--fraud committed by an employee of a business usually begins from a small infraction and escalates. For example, the employee might discover that she accidentally duplicated a paycheck which no one catches. Knowing this, the employee then begins committing other deliberate acts of fraud.
Investigating Hidden Assets
Investigators looking into this (much like a divorce lawyer would look into similar situations) would see the first inadvertent offense as the "date of first indication," before which other instances of malfeasance are unlikely.
Similarly, when your divorce attorney is investigating the possibility of hidden assets by a spouse, the "date of first indication" would be the date on which either party might have begun to hide assets.
Usually, this can be identified as the date on which one or both parties realized the marriage was over. This might be the date that a certain fight happened or the date an affair began (or was discovered).
In other situations, it might be when a gradual realization happened between the partners. It might not even be at the same time for both.
However, the investigation will try to determine when noticeable changes of behavior occurred in you or your spouse, which will give a starting point for uncovering possible hidden assets.