Probably.

First, any ICBM or interceptor is very loud and bright. You can hear (and fell) a Minuteman III ICBM launch from at least 30 miles away. Having witnessed ABM tests as well, they are pretty loud as well.

If you assume whatever country launched at us could completely keep it a secret (unlikely), the US government kept it a secret (very unlikely given the push for more money for missile defense), and some secret space based system that doesn't rely on land based interceptors (really, really unlikely). I guess.

Although, the fissile material from any nuclear weapon (if destroyed) would produce detectable increases radiation anywhere near (within a few hundred miles) where the intercept happened would be easily detectable (if people looked for it).

On a final note - the idea that the US would be attacked by any sort of WMD and not retaliate (even if the WMD was intercepted) is laughable. In fact, one could be reasonably assured that our ICBMs would be in the air before anything even approached the continental US.

About the only scenario I can come up with that could keep this totally invisible would be if it launched from a nuclear sub and then intercepted by a nuclear sub. And then, those subs would still have to be a sea, because once the hit shore...we'd know.

Otherwise, we'd know.

The thermal bloom would be detected by a number of satellite's, anything with infrared sensors would see it. At the sign of a launch a veritable army of people would know within minutes.

First you have the thermal bloom detections from infrared satellites. If we go for a scenario lets take a site outside Shenyang as launch point. It would be detected about 15 seconds post-launch and located as part of a chinese missile complex. This information would be fed back to NORAD where alarms would sound, the ranking official would pick up a gold coloured phone that goes straight to the NMCC in the pentagon. At the same time NORAD would begin putting US Forces on alert status, spinning up ICBM's and notifying launch crews.

60 seconds after launch, 1000+ people know.

US Radar installations begin tracking, Cobra Dane at Shemya and PAVEPAWS installations at Beale AFB, CA begin preparations to track it.

3 minutes.

After that the word begins to filter across Washington, Pentagon notifies POTUS & First Family are giving marching order. Marine One extraction units begin taking off from Andrews AFB. Members of the cabinet, senate & house are prepared to be evacuated.

5 minutes.

President looses control of all ABM's to NORAD who begin to fire at inbound, inbound intercepted off the canadian coast. With an inbound travelling at 14,000 knots per minute this is extremely hard.

Media picks up on evacuation, sources reveal inbound missile. Media goes ape# and warns everyone.

within 10 minutes of a launch its quite possible that everyone with a TV or radio would know.

Theres always a leaker