Whoever did send the letter, is possibly reading this now and maybe, trying to catch you out again - by getting you irate and say something you shouldn't.
As frustrating as it might be and as angry as you possibly are, hang back - bite your lip and don't give anyone ammunition.
Give your job search to your advisor and prove the grass/telltale/busybody wrong.
RE: What's annoying you today? V2
'Illegitimis non carborundum'