Presumably there is nothing about this in your contract or any company policy?
Whast were the circumstances of the disclosure?
If there is no policy then your employer can only rely on it being breach of an implied term in your contract of " trust and confidence".
It may have be wise not to say anything but , as a matter of fact, the engineer had been suspended. Suspension is not the same as doing something wrong it is commonly used by employers to investigate an allegation without the employee having to come into contact with others who may be involved. It also begs the question what are you supposed to have said instead?
I am not sure what you are supposed to have said to explain the engineers absence, if your employers are suggesting you should have lied this hardly places them in a great light either.
I do not believe in all the circumstances this can be gross misconduct which should be reserved for more serious misconduct such as fighting, dishonesty etc.
I hope this helps. If there are any further points please reply.