Like all federal agencies, NCIS Special Agents carry firearms with them at all times as a means of personal protection or in case they are attacked by potential suspects who may be armed if it's in the field or in the office where they work.
Each agent of the NCIS Washington D.C. field office carries a SIG-Sauer P228 which is presumably the standard-issued weapon for all NCIS agents assigned to D.C. The D.C.'s field office counterpart, the NCIS: Los Angeles based Office of Special Projects originally carried the SIG-Sauer P228 before switching to the SIG-Sauer P229.
Nearly all the agents there carry the new SIG-Sauer P229 except for Tech Operator Eric Beale who has expressed no desire to become a field agent. Marty Deeks who is a Detective with the Los Angeles Police Detective and also the NCIS/LAPD Liaison Officer is the only other person not to wield the SIG-Sauer P229, instead choosing to carry a Smith and Wesson 5944 as his main weapon due to the fact that he's still with the LAPD- Los Angeles Police Department and not NCIS.
Whenever their guns aren't being used, the agents carry their weapons in holsters that are on their hips which also means that they're ready to grab them at a moment's notice and also at the first sign of possible trouble.
Even NCIS Directors such as Jennifer Shepard and her successor, Leon Vance carry SIG-Sauers, also as a means of defense as well protecting themselves against their enemies.