How would you know that? Do you have access to the passenger information?
Anyway, were they 744 crew, or perhaps crew destined to operate UK schedules?
This is just the sort idle rumourmongering that will assist in sending Monarch under, along with their employees. Some spotters just won't give up will they? Hopefully your employer may find themselves in the media spotlight - imagine how you'd feel if others were indulging in idle speculation that put your job under threat.
Very easy, they were seen checking in and out of one of the Manchester airport hotels and the sign in sheet clearly said United crew.