I have just checked our epirb (MT400) and noticed that there appears to be cracks in the plastic casing, on the corners, about an inch below the screw holes. One is deep enough to get your finger nail to catch on it and extends around the unit for about 1.25 inches. on the three other corners there is evidence of the same type of cracking and what appears to be similar to stress fractures in grp. None of the other corners though are yet 'feelable' with the finger nail.
hmm..

Have emailed GME in Australia with a picture to ask for advise.