Let's assume the caracters are talking to a stranger and he tells them his true appearance is different from what they see because it is concealed for safety reasons.
Unknown to the charactes, the stranger's true identity has been concealed by a Disguise Self spell.
What does happen at this point?
Do they receive a passive saving throw to recognize his appearance as illusory, even if they have not interacted physically with him during the conversation?
Are they allowed to study him carefully (only by sight and hearing) in order to receive the saving throw, even if they do not know the disguise he is using is an illusion?
As usual with illusions I point to All about illusions. There are also three other parts, just google it and it comes up, but this is the relevant section for our purposes.
In this particular instance I propose the following:
You need to determine how bluntly he says I'm under an illusion.
If he strongly states it the party should get an automatic check at the +4 for being told it's an illusion.
If it's pretty obvious the party members should have to declare they are studying him closely and use a move action and receive the +4 for disbelief.
If he's relatively weak with letting them know they should have to declare they are studying him closely and use a move action. They will not receive a -4 in this instance.
Under all circumstances they should automatically (really part of the same action used to study for an illusion) get a new perception vs his disguise. His disguise may very well still hold up even if they disbelieve an illusion. +10 is not a very good bonus, so I would assume he has ranks in disguise already.
Saving Throws and Illusions (Disbelief): Creatures encountering an illusion usually do not receive saving throws to recognize it as illusory until they study it carefully or interact with it in some fashion.
As you can see you do not need to interact physically. Any interaction will allow a saving throw, as will studying it closely. You do not need to physically touch something to study it.