Back in November, 2012 when I was in the Staten Island area right after hurricane Sandy hit, I noted several UHF FEMA frequencies that had P25 radio traffic right after federal aid started moving into the area. The one frequency that seemed to show the most "hits" was 407.6625, N421.
I'm back in the NYC area and am still hearing a lot of short transmissions on this frequency, but I'm not sure if this is a permanent FEMA repeater in the area, perhaps part of a wide-area FEMA network. I have also heard these same key-ups with no voice on the same frequency in the Atlantic City, NJ area.
The key-ups with no voice may be the repeaters being in a "vote-scan" mode that allows field radios to select the nearest repeater automatically.
UPDATE: Just to clarify, there is encrypted voice traffic on these repeaters, not just voiceless key ups...
UPDATE 2 - On my last two trips to the Atlantic City area, the key ups on 407.6625 are no longer being heard.