Travelling within the Schengen zone with a short-time visa EEA family members visa

I can not find information regarding travel of non-EU nationals who are given a Schengen visas with the remarks stating their status being the family member of an EEA national.

I understand that these visas are issued free of charge and they allow the family members to travel with the EU national.

However, here is what I don't understand:

• Is it necessary for the EU spouse to be physically right next to the non-EU spouse at all times?

• What happens if, say, the non-EU spouse goes on a day trip to see a common friend in a different city alone? If this city is in a border country, there might be check on the train or bus, for instance.

I would just like to understand the legality of what happens once the non-EU family member (say, spouse) is in the Schengen area but without being physically right next to the EU citizen from whom s/he derives his or her right to be there in the first place.