While the Foundation believes that the two creatures are separate entities, they are not necessarily sure.
It is obvious that whatever happened on the 4th exploration caused this entity to move towards the front entrance, whether it was forcibly trapped in there or attempting to lure more people in however is a mystery.
It would have learned that as soon as its presence was revealed the target would retreat back to the surface.
The Foundation has been looking for a proper method of containing these creatures, but despite their best efforts, they are not optimistic.
It is also possible that the Foundation was attempting to contain a creature that simply escaped containment too.
Unauthorized access will be monitored, located and dealt with.