Colt did produce a few 3" shrouded Detective Specials.
These are NOT common, and often bring premium prices due to the low production.
The Custom Shop produced a good number of limited production Colt's over the years, and there were a number of various Detective Special versions.
A common feature of many Custom Shop guns was the Satin Electroless Nickel finish, which does LOOK like stainless steel.
Colt often called this finish "Coltguard".
Also, the Custom Shop will make about anything you can pay for, and many people have Custom Shop guns with special finishes, tuned actions, non-standard barrels, etc.
A 3" NIB Detective Special would be an excellent buy at $400.00, and depending on WHAT the Custom Shop features are, it could be a real steal.
The box should have the Custom Shop sticker and at least some details on the box.
You might want to look closely at this, it sounds like a good one.