DoD must do it differently than Verisign, because I know if I need one of my expired keys to open an old encrypted email, I can go recover it at a certain .mil site. I can see a list of all of the keys I have ever had at that site (back to 2003).
I have done it several times and I help people do it all the time.
But I don't think that would apply to government in general, probably just DoD. Also if some rogue FDA official were covering his/her tracks by encrypting, they certainly would not do it with an escrowed key. They would use a PGP key or a PKI private key that they generate and hang on to (they can also back it up securely, so losing it is not a problem, and thus one of the DoD arguments for escrow is simply a red herring).